altstack: Include config.h in run.c

ccan programs should always include config.h before anything else to make
sure everything is set up correctly.  Doing so in altstack's run.c means
it no longer needs an explicit _XOPEN_SOURCE 700, since _GNU_SOURCE is set
in config.h (for GNU libc, anyway).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/altstack/test/run.c

@@ -1,3 +1,4 @@
+#include "config.h"
 #include <assert.h>
 #include <err.h>
 #include <errno.h>
@@ -8,7 +9,6 @@
 #include <sys/mman.h>
 #include <ccan/tap/tap.h>
 #include <ccan/altstack/altstack.h>
-#define _XOPEN_SOURCE 700
 #include <stdio.h>
 
 enum {