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

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

include/uapi/drm/mga_drm.h

@@ -37,6 +37,10 @@
 
 #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 Xserver file (mga_sarea.h)
  */
@@ -416,4 +420,8 @@ typedef struct drm_mga_getparam {
 	void __user *value;
 } drm_mga_getparam_t;
 
+#if defined(__cplusplus)
+}
+#endif
+
 #endif