user-accounts: fix compilation warning and function name

https://bugzilla.gnome.org/show_bug.cgi?id=704741

panels/user-accounts/pw-utils.h

@@ -29,4 +29,4 @@ gdouble  pw_strength   (const gchar  *password,
                         const gchar **hint,
                         const gchar **long_hints,
                         gint         *strength_level);
-int      pw_strenght_hint_get_width_chars ();
+int      pw_strength_hint_get_width_chars (void);