stubs/mutex: Fix compilation by initializing variable in WrappedMutex class.

This solves the following error in gcc:
explicitly defaulted function cannot be declared 'constexpr' because the implicit declaration is not 'constexpr'

src/google/protobuf/stubs/mutex.h

@@ -125,11 +125,11 @@ class GOOGLE_PROTOBUF_CAPABILITY("mutex") PROTOBUF_EXPORT WrappedMutex {
 
  private:
 #if defined(GOOGLE_PROTOBUF_SUPPORT_WINDOWS_XP)
-  CallOnceInitializedMutex<CriticalSectionLock> mu_;
+  CallOnceInitializedMutex<CriticalSectionLock> mu_ {};
 #elif defined(_MSC_VER)
-  CallOnceInitializedMutex<std::mutex> mu_;
+  CallOnceInitializedMutex<std::mutex> mu_ {};
 #else
-  std::mutex mu_;
+  std::mutex mu_ {};
 #endif
 };