spec: Require pod2man when running autoreconf

Since commit b8a32e0e, all man pages
depend on configure.ac so that they are properly regenerated whenever
libvirt version changes. Thus libvirt.spec needs to have a build
dependency on pod2man when %{enable_autotools} is set.

libvirt.spec.in

@@ -394,6 +394,7 @@ BuildRequires: autoconf
 BuildRequires: automake
 BuildRequires: gettext-devel
 BuildRequires: libtool
+BuildRequires: /usr/bin/pod2man
 %endif
 BuildRequires: python-devel
 %if %{with_systemd}