Release the semaphore in dealloc, fixing leak.

objectivec/GPBMessage.m

@@ -886,6 +886,7 @@ static GPBUnknownFieldSet *GetOrMakeUnknownFields(GPBMessage *self) {
 - (void)dealloc {
   [self internalClear:NO];
   NSCAssert(!autocreator_, @"Autocreator was not cleared before dealloc.");
+  dispatch_release(readOnlySemaphore_);
   [super dealloc];
 }