librt: tests: fix minor issues with higher WARNS

got_sigalrm is a global with external linkage and must therefore have a
previous extern declaration. There's no reason to maintain the status quo
there, so just make it static.

The result var is unused.

This part of the test has not been upstreamed, presumably because it exists
solely for sem_clockwait_np. We should perhaps consider moving it into its
own test file outside of ^/contrib/netbsd-tests, but this can happen later.

MFC after: 1 week