spec: require xen-devel for libxl driver

When using 'rpmbuild --define "_without_xen 1"', but on a new enough
Fedora where %{with_libxl} still gets set to 1 by default, the
build dependencies were incomplete, which could result in 'make rpm'
failing because ./configure failed to build the libxl driver.

* libvirt.spec.in (BuildRequires): Fix xen-devel condition.
Signed-off-by: NEric Blake <eblake@redhat.com>

libvirt.spec.in

@@ -412,7 +412,7 @@ BuildRequires: python-devel
 %if %{with_systemd}
 BuildRequires: systemd-units
 %endif
-%if %{with_xen}
+%if %{with_xen} || %{with_libxl}
 BuildRequires: xen-devel
 %endif
 BuildRequires: libxml2-devel