Add __unused to variable to silence unused-variable warning

NSCAssert() can be optimised away leaving the `firstField` appear as unused.  (See details in the internal CL)

objectivec/GPBUtilities.m

@@ -2248,7 +2248,7 @@ void GPBMaybeClearOneof(GPBMessage *self, GPBOneofDescriptor *oneof,
     NSCAssert([[self descriptor] oneofWithName:oneof.name] == oneof,
               @"OneofDescriptor %@ doesn't appear to be for %@ messages.",
               oneof.name, [self class]);
-    GPBFieldDescriptor *firstField = oneof->fields_[0];
+    GPBFieldDescriptor *firstField __unused = oneof->fields_[0];
     NSCAssert(firstField->description_->hasIndex == oneofHasIndex,
               @"Internal error, oneofHasIndex (%d) doesn't match (%d).",
               firstField->description_->hasIndex, oneofHasIndex);