Avoid compile error on macos mojave

Fixing the following error by adding the number of items in
'enum feerate' to the end of it.

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/chaintopology.c

@@ -313,6 +313,7 @@ const char *feerate_name(enum feerate feerate)
 	case FEERATE_PENALTY: return "penalty";
 	case FEERATE_MIN: return "min_acceptable";
 	case FEERATE_MAX: return "max_acceptable";
+	case NUM_FEERATES: break;
 	}
 	abort();
 }

lightningd/chaintopology.h

@@ -30,8 +30,8 @@ enum feerate {
 	FEERATE_PENALTY,
 	FEERATE_MIN,
 	FEERATE_MAX,
+	NUM_FEERATES
 };
-#define NUM_FEERATES (FEERATE_MAX+1)
 
 /* We keep the last three in case there are outliers (for min/max) */
 #define FEE_HISTORY_NUM 3