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

This repairs make check.

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

utils.c

@@ -1,6 +1,8 @@
 #include "utils.h"
 #include <ccan/str/hex/hex.h>
 
+secp256k1_context *secp256k1_ctx;
+
 char *tal_hexstr(const tal_t *ctx, const void *data, size_t len)
 {
 	char *str = tal_arr(ctx, char, hex_str_size(len));