spec: add exlicit docdir

OpenSUSE does not ship docs in the normal dir and their rpm macro
does not appear to set it for us.

Based on corosync 30c7f3319f003efa1abeb2173eef9e5c7943ae5b.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>

corosync-qdevice.spec.in

@@ -70,7 +70,8 @@ BuildRequires: autoconf automake libtool
 	--enable-qdevices \
 	--enable-qnetd \
 	--with-initddir=%{_initrddir} \
-	--with-systemddir=%{_unitdir}
+	--with-systemddir=%{_unitdir} \
+	--docdir=%{_docdir}
 
 make %{_smp_mflags}