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/wire_error.c

@@ -1,3 +1,4 @@
+#include "config.h"
 #include <ccan/mem/mem.h>
 #include <ccan/tal/str/str.h>
 #include <common/type_to_string.h>