Remove inclusion of Xlib headers in Cogl headers

Xlib headers define many trivially named objects which can later cause
name collision problems when only cogl.h header is included in a program
or library. Xlib headers are now only included through including the
standalone header cogl-xlib.h.

https://bugzilla.gnome.org/show_bug.cgi?id=661174

Reviewed-by: Robert Bragg <robert@linux.intel.com>

cogl/cogl-renderer.c

@@ -45,6 +45,7 @@
 #include "cogl-winsys-stub-private.h"
 #include "cogl-winsys-egl-private.h"
 #include "cogl-config-private.h"
+#include "cogl-xlib-renderer.h"
 
 #ifdef COGL_HAS_GLX_SUPPORT
 extern const CoglWinsysVtable *_cogl_winsys_glx_get_vtable (void);