Remove Xfuncproto.h and XKB.h from xkbcommon/xkbcommon.h

Remove Xfuncproto.h and XKB.h from xkbcommon/xkbcommon.h

The kbproto header is already not needed here anymore.

Move the _X_EXPORT's to the corresponding function definitions, and use
straight extern "C" clauses instead of _XFUNCPROTOBEGIN/END.

It also makes more sense to have the EXPORT's in the source files, as it
provides some documentation to the reader, whereas in the header it's
obvious.

Signed-off-by: Ran Benita <ran234@gmail.com>
[daniels: Updated for xkb_keymap changes.]