ppp: make prototypes more valid

So they pass -Wstrict-prototypes which we aren't using but might
some day.

src/ppp-manager/nm-pppd-plugin.c

@@ -240,13 +240,13 @@ nm_ip_up (void *data, int arg)
 }
 
 static int
-get_chap_check()
+get_chap_check (void)
 {
 	return 1;
 }
 
 static int
-get_pap_check()
+get_pap_check (void)
 {
 	return 1;
 }