build: include <config.h> for kill(2), isblank(3), and isascii(3)

config.h defines _GNU_SOURCE, which in turn defines the bits necessary
for kill, isblank, and isascii.  So wherever we use those, we need
to make sure config.h is included.

system-settings/plugins/ifnet/net_utils.c

@@ -19,6 +19,7 @@
  * Copyright (C) 1999-2010 Gentoo Foundation, Inc.
  */
 
+#include <config.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>