debian/rules: explicitly enable building manpages

This fixes a build failure in dh_installwms.

fixes #4136

debian/rules

@@ -20,7 +20,7 @@ override_dh_autoreconf:
 
 override_dh_auto_configure:
 	# Set -Ddocdir; the default is /usr/share/doc/i3
-	dh_auto_configure -- -Ddocdir=/usr/share/doc/i3-wm
+	dh_auto_configure -- -Ddocdir=/usr/share/doc/i3-wm -Dmans=true
 
 override_dh_builddeb:
 	# bintray does not support xz currently.