tests: don't mock the time() function on mingw

tests: don't mock the time() function on mingw

The mingw header define time() as a static inline function and this
causes a duplicate definition build failure. Since we're not using the
LD_PRELOAD at all on Mingw, we ideally wouldn't compile any of the
mock libraries. Rather than change the build system now though, this
just stubs out the offending function.

Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>