libc: LOCK() and UNLOCK() do not have to return a value

The return values of LOCK() and UNLOCK() are never used. Besides, in the
current Musl version these macros do not have return values at all.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>