build: libcheck may require linking against rt

build: libcheck may require linking against rt

In static linking scenarios, this is required to avoid this error
building tests:

/work/prefix/lib/libgstcheck-1.0.a(check_run.c.o): In function `tcase_run_tfun_fork':
/work/gstreamer/_builddir/../../../src/gstreamer/libs/gst/check/libcheck/check_run.c:476: undefined reference to `timer_create'
/work/gstreamer/_builddir/../../../src/gstreamer/libs/gst/check/libcheck/check_run.c:483: undefined reference to `timer_settime'
/work/gstreamer/_builddir/../../../src/gstreamer/libs/gst/check/libcheck/check_run.c:493: undefined reference to `timer_delete'
/work/prefix/lib/libgstcheck-1.0.a(check.c.o): In function `check_get_clockid':
/work/gstreamer/_builddir/../../../src/gstreamer/libs/gst/check/libcheck/check.c:628: undefined reference to `timer_create'
/work/gstreamer/_builddir/../../../src/gstreamer/libs/gst/check/libcheck/check.c:629: undefined reference to `timer_delete'

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/447>