Add missing #include <pthread.h>

In file included from /jhbuild/checkout/xorg/xserver/hw/xwin/winSetAppUserModelID.c:31:0:
/jhbuild/checkout/xorg/xserver/hw/xwin/winwindow.h:140:11: error: expected declaration specifiers or ‘...’ before ‘pthread_t’
/jhbuild/checkout/xorg/xserver/hw/xwin/winwindow.h:141:11: error: expected declaration specifiers or ‘...’ before ‘pthread_t’
/jhbuild/checkout/xorg/xserver/hw/xwin/winwindow.h:142:11: error: expected declaration specifiers or ‘...’ before ‘pthread_mutex_t’

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>