remove attribute where it does not apply
remove attribute where it does not apply

```
gflags.h(226): warning: attribute "visibility" does not apply here
```

The visibility attribute applies to symbols like functions and variables. A definition of an enumeration type that doesn't contain a variable name doesn't create any symbols.
gflags/gflags,gflags/gflags,gflags/gflags,gflags/gflags