add extern "C" in ff-mac-callback.h

openair2/LAYER2/MAC/ff-mac-callback.h

 #ifndef FF_MAC_CALLBACK_H
 #define FF_MAC_CALLBACK_H
 
+#if defined (__cplusplus)
+extern "C" {
+#endif
+
 /* these are the callback function types from the scheduler to openair */
 
 /* sched */
@@ -16,4 +20,8 @@ typedef void (CschedUeReleaseCnf_callback_t)(void *callback_data, const struct C
 typedef void (CschedUeConfigUpdateInd_callback_t)(void *callback_data, const struct CschedUeConfigUpdateIndParameters *params);
 typedef void (CschedCellConfigUpdateInd_callback_t)(void *callback_data, const struct CschedCellConfigUpdateIndParameters *params);
 
+#if defined (__cplusplus)
+}
+#endif
+
 #endif /* FF_MAC_CALLBACK_H */