locking: Enable RT_MUTEXES by default on PREEMPT_RT.

The CONFIG_RT_MUTEXES option is enabled by CONFIG_FUTEX and CONFIG_I2C.
If both are disabled then a CONFIG_PREEMPT_RT build fails to compile.
It is not possible to have a PREEMPT_RT kernel without RT_MUTEX support
because RT_MUTEX based locking is always used.

Enable CONFIG_RT_MUTEXES by default on PREEMPT_RT builds.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YgKmhjkcuqWXdUjQ@linutronix.de