rtmutex: Make the rtmutex tester depend on BROKEN

It has been broken for quite some time. Just the recent updates made
it compile time broken. Make it depend on BROKEN instead of removing
it right away as we want a proper replacement.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

lib/Kconfig.debug

@@ -835,7 +835,7 @@ config DEBUG_RT_MUTEXES
 
 config RT_MUTEX_TESTER
 	bool "Built-in scriptable tester for rt-mutexes"
-	depends on DEBUG_KERNEL && RT_MUTEXES
+	depends on DEBUG_KERNEL && RT_MUTEXES && BROKEN
 	help
 	  This option enables a rt-mutex tester.