kdrive/modesetting: use the drm_defines.h GBM fallbacks

ms_gbm.c uses GBM_BO_USE_FRONT_RENDERING, which older libgbm versions
accepted by meson don't provide, so Xmodesetting fails to build against
them. Include drm_defines.h, which defines it to 0 in that case, as the
Xorg modesetting driver does.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: galternat <galternat.twins223@simplelogin.com>

hw/kdrive/modesetting/ms_gbm.c

@@ -9,6 +9,8 @@
 
 #include <drm_fourcc.h>
 
+#include "drm_defines.h"
+
 /* TODO: Stuff that should be in msutil */
 
 typedef struct {