[Xcb] xcb_atom.h with missing c++ facilities

[comick]

I'm using xcb_atom.h in a C++ project.
I had problems with the linking phase and after a research i fixed the problem 
with something like:

#ifdef __cplusplus
extern "C" {
#endif
#include <xcb/xcb_atom.h>
#ifdef __cplusplus
}
#endif

extern is present in all other xcb utils packages, so i think should be added 
to xcb_atom too.

Hope this help.