gallium/state_trackers/egl: use X11_INCLUDES rather than X11_CFLAGS

gallium/state_trackers/egl: use X11_INCLUDES rather than X11_CFLAGS

The X11_CFLAGS variable is undefined (not defined in config.status).
It appears the intent was to use X11_INCLUDES defined in configure.ac.
It is used for building the code in the x11 subdir.

The build does not fail on this one as LIBDRM_CFLAGS happens to have
the inludedir value as the one for X11. It will not always be the case.
The option --enable-gallium-egl is required durimg configuration.

Acked-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>