futex: Make futex_private_hash_get() static

futex_private_hash_get() is not used outside if its compilation unit.
Make it static.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250710110011.384614-4-bigeasy@linutronix.de