mesa: Remove GLXContextID typedef from glxext.h.

mesa: Remove GLXContextID typedef from glxext.h.

This patch fixes this build error with gcc <= 4.5 and clang <= 3.1.

  CC     clientattrib.lo
In file included from ../../include/GL/glx.h:333:0,
                 from glxclient.h:45,
                 from clientattrib.c:32:
../../include/GL/glxext.h:275:13: error: redefinition of typedef 'GLXContextID'
../../include/GL/glx.h:171:13: note: previous declaration of 'GLXContextID' was here

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70591
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>