storage: remove use of stat-time.h headers

storage: remove use of stat-time.h headers

The gnulib stat-time.h header provides wrapper functions
to hide the differences in 'struct stat' between various
platforms.

Linux and FreeBSD support the same names, except for
birthtime which Linux only provides in the new 'statx()'
syscall we're not using. macOS has completely different
naming. Since we only rely on this code in one place
we just use conditionals at time of need.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>