supplicant: fix memory corruption with wrong argument to NM_SUPPLICANT_INTERFACE_GROUP_FORMATION_FAILURE signal

The signal is unused (and should be removed).

Still, the parameter passed to g_signal_emit() is a C string, not a
GVariant. I think as there are no subscribers, glib wouldn't actually
do anything with the arguments. Though, I am not sure whether glib still
tries to initialize a GValue with a GVariant type, leading to a crash.

Fixes: f05b7a78c9 ('supplicant: Track P2P Group information, creation and destruction')
(cherry picked from commit c106008091)
(cherry picked from commit 26d6ac5385)
(cherry picked from commit dc9322c0a9)

src/supplicant/nm-supplicant-interface.c

@@ -2981,5 +2981,5 @@ nm_supplicant_interface_class_init (NMSupplicantInterfaceClass *klass)
 	                  G_SIGNAL_RUN_LAST,
 	                  0,
 	                  NULL, NULL, NULL,
-	                  G_TYPE_NONE, 1, G_TYPE_VARIANT);
+	                  G_TYPE_NONE, 1, G_TYPE_STRING);
 }