Build: configure: Use REQUIRE_FUNC for sched_{get,set}scheduler.

We've been searching for these functions in librt, but
lib/services/Makefile.am (the only library that uses these functions)
doesn't add -lrt to the linker flags anywhere. Thus, any platform where
these functions would be found in librt would fail to build. We can
therefore get rid of the search and just use REQUIRE_FUNC.

Related T876