x11/virglrenderer: unbreak with libglvnd

In file included from ../src/vrend_blitter.c:48:
In file included from ../src/vrend_renderer.h:34:
../src/virgl_gbm.h:28:10: fatal error: 'gbm.h' file not found
#include <gbm.h>

^~~~~~~

Reported by: antoine (via bug 246767 exp-run)