rtmutex-tester: make it build without BKL

The big kernel lock is going away, so make sure
that if it is disabled by Kconfig, we do not
try to validate it, which would result in
compile errors.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>