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

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

include/uapi/drm/via_drm.h

@@ -26,6 +26,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 /* WARNING: These defines must be the same as what the Xserver uses.
  * if you change them, you must change the defines in the Xserver.
  */
@@ -271,4 +275,8 @@ typedef struct drm_via_dmablit {
 	drm_via_blitsync_t sync;
 } drm_via_dmablit_t;
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif				/* _VIA_DRM_H_ */