posix: semaphore: explicitly ignore return of k_mutex_unlock()

The nsem_list_unlock() static inline function looks as though it
was designed to always ignore the return value of
k_mutex_unlock(), presumably for performance reasons.

A quick audit of the code looks as though the call should
always succeed.

Prepend (void) to the call to avoid the coverity defect in the
future.

CID 444386

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>