Resolve Swift warnings by specifying void arguments
Resolve Swift warnings by specifying void arguments

The following functions in the `fork.h` file cause a `This function declaration is not a prototype` warning in Swift:
```
void grpc_prefork(void);

void grpc_postfork_parent(void);

void grpc_postfork_child(void);

void grpc_fork_handlers_auto_register(void);
```

Explicitly specifying `void` as the argument resolves the warnings.

Reproducible using Xcode 9.2 with `SwiftGRPC`/`gRPC-Core` via CocoaPods.

nicolasnoble/grpc,Vizerai/grpc,grpc/grpc,ncteisen/grpc,muxi/grpc,mehrdada/grpc,nicolasnoble/grpc,dgquintas/grpc,vjpai/grpc,jboeuf/grpc,jtattermusch/grpc,vjpai/grpc,ctiller/grpc,sreecha/grpc,ctiller/grpc,ncteisen/grpc,jboeuf/grpc,carl-mastrangelo/grpc,jtattermusch/grpc,grpc/grpc,donnadionne/grpc,chrisdunelm/grpc,donnadionne/grpc,jboeuf/grpc,donnadionne/grpc,mehrdada/grpc,muxi/grpc,jtattermusch/grpc,muxi/grpc,sreecha/grpc,mehrdada/grpc,ctiller/grpc,thinkerou/grpc,simonkuang/grpc,sreecha/grpc,mehrdada/grpc,vjpai/grpc,jboeuf/grpc,mehrdada/grpc,sreecha/grpc,pszemus/grpc,vjpai/grpc,mehrdada/grpc,firebase/grpc,ncteisen/grpc,nicolasnoble/grpc,jtattermusch/grpc,donnadionne/grpc,vjpai/grpc,simonkuang/grpc,firebase/grpc,ejona86/grpc,nicolasnoble/grpc,vjpai/grpc,pszemus/grpc,carl-mastrangelo/grpc,Vizerai/grpc,ctiller/grpc,ncteisen/grpc,ejona86/grpc,dgquintas/grpc,sreecha/grpc,chrisdunelm/grpc,grpc/grpc,mehrdada/grpc,stanley-cheung/grpc,ctiller/grpc,pszemus/grpc,dgquintas/grpc,ctiller/grpc,mehrdada/grpc,Vizerai/grpc,grpc/grpc,donnadionne/grpc,jtattermusch/grpc,carl-mastrangelo/grpc,jtattermusch/grpc,ejona86/grpc,pszemus/grpc,carl-mastrangelo/grpc,ctiller/grpc,dgquintas/grpc,carl-mastrangelo/grpc,ejona86/grpc,firebase/grpc,stanley-cheung/grpc,ejona86/grpc,carl-mastrangelo/grpc,dgquintas/grpc,ncteisen/grpc,carl-mastrangelo/grpc,ctiller/grpc,thinkerou/grpc,chrisdunelm/grpc,carl-mastrangelo/grpc,grpc/grpc,grpc/grpc,ejona86/grpc,grpc/grpc,dgquintas/grpc,vjpai/grpc,jboeuf/grpc,sreecha/grpc,Vizerai/grpc,Vizerai/grpc,jboeuf/grpc,grpc/grpc,nicolasnoble/grpc,stanley-cheung/grpc,nicolasnoble/grpc,chrisdunelm/grpc,donnadionne/grpc,nicolasnoble/grpc,jtattermusch/grpc,thinkerou/grpc,thinkerou/grpc,mehrdada/grpc,simonkuang/grpc,thinkerou/grpc,pszemus/grpc,jboeuf/grpc,vjpai/grpc,sreecha/grpc,ctiller/grpc,sreecha/grpc,chrisdunelm/grpc,muxi/grpc,Vizerai/grpc,ctiller/grpc,thinkerou/grpc,simonkuang/grpc,jtattermusch/grpc,stanley-cheung/grpc,nicolasnoble/grpc,pszemus/grpc,jtattermusch/grpc,chrisdunelm/grpc,thinkerou/grpc,grpc/grpc,stanley-cheung/grpc,muxi/grpc,mehrdada/grpc,dgquintas/grpc,dgquintas/grpc,ncteisen/grpc,stanley-cheung/grpc,pszemus/grpc,Vizerai/grpc,firebase/grpc,Vizerai/grpc,firebase/grpc,chrisdunelm/grpc,jboeuf/grpc,jtattermusch/grpc,thinkerou/grpc,Vizerai/grpc,jtattermusch/grpc,donnadionne/grpc,chrisdunelm/grpc,ncteisen/grpc,Vizerai/grpc,jboeuf/grpc,grpc/grpc,muxi/grpc,chrisdunelm/grpc,nicolasnoble/grpc,nicolasnoble/grpc,donnadionne/grpc,ejona86/grpc,firebase/grpc,dgquintas/grpc,ejona86/grpc,jboeuf/grpc,donnadionne/grpc,stanley-cheung/grpc,ejona86/grpc,mehrdada/grpc,ctiller/grpc,simonkuang/grpc,firebase/grpc,dgquintas/grpc,firebase/grpc,muxi/grpc,stanley-cheung/grpc,grpc/grpc,vjpai/grpc,muxi/grpc,thinkerou/grpc,vjpai/grpc,carl-mastrangelo/grpc,thinkerou/grpc,donnadionne/grpc,stanley-cheung/grpc,dgquintas/grpc,ejona86/grpc,carl-mastrangelo/grpc,ejona86/grpc,sreecha/grpc,jboeuf/grpc,simonkuang/grpc,muxi/grpc,simonkuang/grpc,chrisdunelm/grpc,stanley-cheung/grpc,ncteisen/grpc,ctiller/grpc,ncteisen/grpc,ncteisen/grpc,chrisdunelm/grpc,stanley-cheung/grpc,donnadionne/grpc,vjpai/grpc,donnadionne/grpc,carl-mastrangelo/grpc,simonkuang/grpc,firebase/grpc,grpc/grpc,sreecha/grpc,ncteisen/grpc,thinkerou/grpc,carl-mastrangelo/grpc,pszemus/grpc,stanley-cheung/grpc,thinkerou/grpc,Vizerai/grpc,mehrdada/grpc,nicolasnoble/grpc,ejona86/grpc,sreecha/grpc,simonkuang/grpc,pszemus/grpc,muxi/grpc,jtattermusch/grpc,firebase/grpc,nicolasnoble/grpc,pszemus/grpc,pszemus/grpc,pszemus/grpc,ncteisen/grpc,sreecha/grpc,firebase/grpc,firebase/grpc,vjpai/grpc,muxi/grpc,muxi/grpc,jboeuf/grpc