configure: use valid args testing sem_timedwait

The sem_timedwait function has been annotated as requiring
non-null args in latest header files from GCC snapshot
representing the future 2.30 release.

This causes configure to fail when -Werror is used:

config-temp/qemu-conf.c: In function ‘main’:
config-temp/qemu-conf.c:2:25: error: null argument where non-null required (argument 1) [-Werror=nonnull]
    2 | int main(void) { return sem_timedwait(0, 0); }
      |                         ^~~~~~~~~~~~~
config-temp/qemu-conf.c:2:25: error: null argument where non-null required (argument 2) [-Werror=nonnull]

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20190617114114.24897-1-berrange@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>