Avoid compile error on macos mojave

Fixing the following error by changing 'enum feerate' to int.

lightningd/bitcoind.c:183:29: error: result of comparison of constant
8 with expression of type 'enum feerate' is always true [-Werror,
-Wtautological-constant-out-of-range-compare]
        for (enum feerate f = 0; f < NUM_FEERATES; f++) {

Changelog-Fixed: compile error on macos

lightningd/bitcoind.c

@@ -181,7 +181,7 @@ static void estimatefees_callback(const char *buf, const jsmntok_t *toks,
 				     "estimatefees",
 				     "bad 'result' field");
 
-	for (enum feerate f = 0; f < NUM_FEERATES; f++) {
+	for (int f = 0; f < NUM_FEERATES; f++) {
 		feeratetok = json_get_member(buf, resulttok, feerate_name(f));
 		if (!feeratetok)
 			bitcoin_plugin_error(call->bitcoind, buf, toks,