FIX: Compilation warning with GCC regarding a non-initialised member from MutexBase class.

googletest/include/gtest/internal/gtest-port.h

@@ -2093,7 +2093,7 @@ class MutexBase {
 // This allows initialization to work whether pthread_t is a scalar or struct.
 // The flag -Wmissing-field-initializers must not be specified for this to work.
 #  define GTEST_DEFINE_STATIC_MUTEX_(mutex) \
-     ::testing::internal::MutexBase mutex = { PTHREAD_MUTEX_INITIALIZER, false }
+     ::testing::internal::MutexBase mutex = { PTHREAD_MUTEX_INITIALIZER, false, 0 }
 
 // The Mutex class can only be used for mutexes created at runtime. It
 // shares its API with MutexBase otherwise.