utils: add a global secp, fix wire to use it.

This repairs make check.

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

utils.h

@@ -3,6 +3,9 @@
 #include "config.h"
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/tal.h>
+#include <secp256k1.h>
+
+extern secp256k1_context *secp256k1_ctx;
 
 /* Allocate and fill in a hex-encoded string of this data. */
 char *tal_hexstr(const tal_t *ctx, const void *data, size_t len);