build: fix building error when building without libvirtd

build: fix building error when building without libvirtd

When building libvirt without libvirtd, I receive the following errors:
make[1]: Leaving directory `/home/wency/source/test/libvirt/src'
 (cd daemon && make  top_distdir=../libvirt-0.8.8 distdir=../libvirt-0.8.8/daemon \
     am__remove_distdir=: am__skip_length_check=: am__skip_mode_fix=: distdir)
make[1]: Entering directory `/home/wency/source/test/libvirt/daemon'
make[1]: *** No rule to make target `libvirtd.8.in', needed by `distdir'.  Stop.

This bug was caused by commit 6db98a2d.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Eric Blake <eblake@redhat.com>