Fix compilation with gdbjit=on.

src/gdb-jit.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 108 matching lines @@
 
   static void AddCode(CodeTag tag, String* name, Code* code);
 
   static void AddCode(CodeTag tag, const char* name, Code* code);
 
   static void AddCode(CodeTag tag, Code* code);
 
   static void RemoveCode(Code* code);
 
   static void RegisterDetailedLineInfo(Code* code, GDBJITLineInfo* line_info);
+
+ private:
+  static Mutex* mutex_;
 };
 
 #define GDBJIT(action) GDBJITInterface::action
 
 } }   // namespace v8::internal
 #else
 #define GDBJIT(action) ((void) 0)
 #endif
 
 #endif