Fix type to be consistent with intel-media-driver

diff -rup libavutil/hwcontext_vaapi.c.orig libavutil/hwcontext_vaapi.c
--- a/libavutil/hwcontext_vaapi.c
+++ b/libavutil/hwcontext_vaapi.c
@@ -1203,7 +1203,7 @@ static int vaapi_map_from_drm(AVHWFrames
 
     if (!use_prime2 || vas != VA_STATUS_SUCCESS) {
         int k;
-        unsigned long buffer_handle;
+        uintptr_t buffer_handle;
         VASurfaceAttribExternalBuffers buffer_desc;
         VASurfaceAttrib buffer_attrs[2] = {
             {