sd-daemon.c: allow use of -lrt to be avoided

In recent glibc, many commonly-used librt functions have moved
from librt to libc. This results in dbus' configure.ac
concluding that we don't need to link in librt. However,
sd-daemon.c needs it for mq_getattr(), causing the build
to fail.

dbus doesn't use POSIX message queues, so I'd prefer to be
able to avoid "if on Linux, link librt for sd-daemon.c".