Makefile: Don't build the srpms repository by default

Currently the Debian and RPM builds share the rpms
and srpms targets, and the srpms_repo rule tries to
run createrepo which is not appropriate and will fail
on Debian.

The srpms_repo target is a convenience, not needed for
building binary packages.   It will return after we
separate the targets used for building .debs and .rpms.

Signed-off-by: Euan Harris <euan.harris@citrix.com>