Add the missing --enable-rhev definition to configure.ac

This fixes an error reported by autogen.sh when
--enable-rhev was on the command line.

Change-Id: I3b253be2710bc979e3e14f1525f0157a0e65440f
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35796
Reviewed-by: Dan Kenigsberg <danken@redhat.com>