race tests: fix unsupported condition

Should be true if single thread is defined, then its not
supported.

Author: 0xc0170

TESTS/mbed_drivers/race_test/main.cpp

@@ -22,7 +22,7 @@
 #include "SingletonPtr.h"
 #include <stdio.h>
 
-#ifndef MBED_RTOS_SINGLE_THREAD
+#ifdef MBED_RTOS_SINGLE_THREAD
   #error [NOT_SUPPORTED] test not supported for single threaded enviroment
 #endif