drm/qxl: add extern C guard for the UAPI header

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

include/uapi/drm/qxl_drm.h

@@ -26,6 +26,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 /* Please note that modifications to all structs defined here are
  * subject to backwards-compatibility constraints.
  *
@@ -147,4 +151,8 @@ struct drm_qxl_alloc_surf {
 	DRM_IOWR(DRM_COMMAND_BASE + DRM_QXL_ALLOC_SURF,\
 		struct drm_qxl_alloc_surf)
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif