po/Makefile.am: use SOURCE_DATE_EPOCH to set POT creation date

po/Makefile.am: use SOURCE_DATE_EPOCH to set POT creation date

When building RPMs for libvirt the PO files are part of libvirt-libs
package.  Now that we generate libvirt.pot during build time the POT
creation date is also generated at that time.

The issue here is that when building libvirt-libs for x86_64 and i686
the generated libvirt.pot file will have different POT creation date
which affects installed PO files as well which leads to conflict when
installing both x86_64 and i686 packages.

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