drm: fix inclusion of drm.h in virtgpu_drm.h

Using `#include "drm.h"` instead of `#include <drm/drm.h>` allow drm
headers to be moved in another directory without changes, like for the
libdrm imports.

Signed-off-by: Gabriel Laskar <gabriel@lse.epita.fr>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
CC: Emil Velikov <emil.l.velikov@gmail.com>
CC: Mikko Rapeli <mikko.rapeli@iki.fi>

include/uapi/drm/virtgpu_drm.h

@@ -24,8 +24,7 @@
 #ifndef VIRTGPU_DRM_H
 #define VIRTGPU_DRM_H
 
-#include <stddef.h>
+#include "drm.h"
-#include "drm/drm.h"
 
 /* Please note that modifications to all structs defined here are
  * subject to backwards-compatibility constraints.