Fixed build using Xcode 10 beta 1 (#231)

third_party/objective_c/google_api_objectivec_client_for_rest/Source/Objects/GTLRObject.m:405:28: error: implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'id _Nonnull' [-Werror,-Wnullable-to-nonnull-conversion]
          [array addObject:@(propName)];
                           ^

Source/Objects/GTLRObject.m

@@ -402,7 +402,7 @@ + (NSMutableArray *)allDeclaredProperties {
         const char *dynamicMarker = strstr(attr, ",D");
         if (dynamicMarker &&
             (dynamicMarker[2] == 0 || dynamicMarker[2] == ',' )) {
-          [array addObject:@(propName)];
+          [array addObject:(id _Nonnull)@(propName)];
         }
       }
       free(properties);