drm: add extern C guard for the UAPI headers

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

include/uapi/drm/drm_mode.h

@@ -29,6 +29,10 @@
 
 #include "drm.h"
 
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
 #define DRM_DISPLAY_INFO_LEN	32
 #define DRM_CONNECTOR_NAME_LEN	32
 #define DRM_DISPLAY_MODE_LEN	32
@@ -623,4 +627,8 @@ struct drm_mode_destroy_blob {
 	__u32 blob_id;
 };
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif