spec: Make the build reproducible

spec: Make the build reproducible

If the spec file applies a patch which touches any file in the API XMLs
dependency tree, we need to regenerate the XMLs and consequently
recreate hvsupport.html. The file will contain a time stamp in a comment
which means it will be different every time the package is built. The
commit a54c9622860 which added the time stamp also added support for
SOURCE_DATE_EPOCH environment variable. Let's set it to the time stamp
of the spec file itself to make the build reproducible.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Pino Toscano <ptoscano@redhat.com>