check-includes: allow redundant "config.h"

We should actually be including this (as it may define _GNU_SOURCE
etc) before any system headers.  But where we include <assert.h> we
often didn't, because check-includes would complain that the headers
included it too.

Weaken that check, and include config.h in C files before assert.h.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/onion.c

@@ -1,3 +1,4 @@
+#include "config.h"
 #include <assert.h>
 #include <ccan/mem/mem.h>
 #include <ccan/opt/opt.h>