force rebuild for theme change

For now, a rebuild is the only way to ensure the changed theme is used.
Ikiwiki normally will not realize style.css has changed, since themes
tend to have the same timestamp for the file.

IkiWiki/Plugin/theme.pm

@@ -22,7 +22,7 @@ sub getsetup () {
 			example => "actiontabs",
 			description => "name of theme to enable",
 			safe => 1,
-			rebuild => 0,
+			rebuild => 1,
 		},
 }