BUG-2707 fix unref'd var

indra/llcommon/llsys.cpp

@@ -1398,6 +1398,8 @@ class FrameWatcher
         }
         LL_CONT << std::fixed << std::setprecision(1) << framerate << '\n'
                 << LLMemoryInfo() << LL_ENDL;
+#else
+	(void)prevSize;
 #endif
 
         return false;