locking/rtmutex: Move debug functions as inlines into common header

There is no value in having two header files providing just empty stubs and
a C file which implements trivial debug functions which can just be inlined.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210326153944.052454464@linutronix.de