Move the ww_mutex definitions into the ww_mutex specific header where they
belong.

Preparatory change to allow compiling ww_mutexes standalone.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211304.110216293@linutronix.de