rpm: add dep on xfsprogs-devel for reflink support

Support for XFS reflink clone was added in:

  commit 8ed874b3
  Author: Julio Faracco <jcfaracco@gmail.com>
  Date:   Fri Jul 6 10:43:01 2018 -0300

    storage: Rename btrfsCloneFile to support other filesystems.

  commit 2e11298f
  Author: Julio Faracco <jcfaracco@gmail.com>
  Date:   Fri Jul 6 10:43:00 2018 -0300

    configure: Adding XFS library/headers check.

But these patches missed that the xfs/xfs.h header is not installed
unless you have xfsprogs-devel present.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

libvirt.spec.in

@@ -326,6 +326,8 @@ BuildRequires: libiscsi-devel
 BuildRequires: parted-devel
 # For Multipath support
 BuildRequires: device-mapper-devel
+# For XFS reflink clone support
+BuildRequires: xfsprogs-devel
 %if %{with_storage_rbd}
 BuildRequires: librados2-devel
 BuildRequires: librbd1-devel