build: avoid stranded polkit file in client-only build

build: avoid stranded polkit file in client-only build

daemon/Makefile.am installs a .policy file if WITH_LIBVIRTD and
WITH_POLKIT are both set.  src/Makefile.am, on the other hand,
installs a .policy file if WITH_POLKIT1 is set, but without checking
WITH_LIBVIRTD.  When running 'make rpm' with client_only manually
set, on a Fedora 19 box, that leads to a failure:

RPM build errors:
    Installed (but unpackaged) file(s) found:
   /usr/share/polkit-1/actions/org.libvirt.api.policy

Fix it by adding another conditional.

* src/Makefile.am (polkitaction_DATA): Make conditional.

Signed-off-by: Eric Blake <eblake@redhat.com>