Remove unnecessary header including to get rid of warning message (#637)
Remove unnecessary header including to get rid of warning message (#637)

This patch will get rid of following warning message in compiling.

```
compiling ../../../../ext/RMagick/rmagick.c
In file included from /home/travis/.rvm/rubies/ruby-2.3.8/include/ruby-2.3.0/ruby/ruby.h:1988:0,
                 from /home/travis/.rvm/rubies/ruby-2.3.8/include/ruby-2.3.0/ruby.h:33,
                 from ../../../../ext/RMagick/rmagick.h:28,
                 from ../../../../ext/RMagick/rmagick.c:13:
/home/travis/.rvm/rubies/ruby-2.3.8/include/ruby-2.3.0/ruby/intern.h:923:29: warning: ‘struct timespec’ declared inside parameter list
 void rb_timespec_now(struct timespec *);
```
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick