libvit.spec.in: Add missing BuildRequires against sheepdog

We were adding a sheepdog requirement at runtime, but forgetting
to turn it on at build time, so the underlying code was never
built.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

libvirt.spec.in

@@ -357,6 +357,9 @@ BuildRequires: ceph-devel
 BuildRequires: glusterfs-api-devel >= 3.4.1
 BuildRequires: glusterfs-devel >= 3.4.1
 %endif
+%if %{with_storage_sheepdog}
+BuildRequires: sheepdog
+%endif
 %if %{with_numactl}
 # For QEMU/LXC numa info
 BuildRequires: numactl-devel