Fix memory leak for battery tooltips

Instead of returning a const string, that is fed into strdup, tooltip
functions are now supposed to return allocated strings. This fixes a
memory leak in the battery tooltip.

This is used instead of simply freeing the memory in the battery
tooltip function, since it also avoids a uselesss strdup().

src/battery/linux.c

@@ -319,7 +319,7 @@ static gchar* power_human_readable(struct psy_battery *bat) {
 	}
 }
 
-const char* linux_batteries_get_tooltip() {
+char* linux_batteries_get_tooltip() {
 	GList *l;
 	GString *tooltip = g_string_new("");
 	gchar *result;