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

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

include/uapi/drm/radeon_drm.h

@@ -35,6 +35,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 /* WARNING: If you change any of these defines, make sure to change the
  * defines in the X server file (radeon_sarea.h)
  */
@@ -1067,4 +1071,8 @@ struct drm_radeon_info {
 
 #define CIK_TILE_MODE_DEPTH_STENCIL_1D		5
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif