xl2tpd: fix a minor compiler warning

gcc complains about atexit() being implicitely defined in
xl2tpd-control.c
Fix that by including stdlib.h in xl2tpd-control.c

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

net/xl2tpd/patches/250-xl2tpd-control-atexit-needs-stdlib.patch

@@ -0,0 +1,12 @@
+Index: xl2tpd-1.3.7pre20141126/xl2tpd-control.c
+===================================================================
+--- xl2tpd-1.3.7pre20141126.orig/xl2tpd-control.c
++++ xl2tpd-1.3.7pre20141126/xl2tpd-control.c
+@@ -12,6 +12,7 @@
+  
+ #define _GNU_SOURCE
+ 
++#include <stdlib.h>
+ #include <stdio.h>
+ #include <string.h>
+ #include <stdarg.h>