check: make sure all files outside contrib/ include "config.h" first.

And turn "" includes into full-path (which makes it easier to put
config.h first, and finds some cases check-includes.sh missed
previously).

config.h sets _GNU_SOURCE which really needs to be done before any
'#includes': we mainly got away with it with glibc, but other platforms
like Alpine may have stricter requirements.

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

common/decode_array.c

@@ -1,3 +1,4 @@
+#include "config.h"
 #include <ccan/cast/cast.h>
 #include <common/decode_array.h>
 #include <wire/peer_wire.h>