build: fix unused function error

so far, this has only been seen with clang 10.0.0-4ubuntu1

Using different compilers never disappoints...

src/fellow_log.c

@@ -4627,7 +4627,7 @@ fellow_log_ban_keep(struct flivs *flivs, const struct fellow_dle *e,
 	return (0);
 }
 
-#ifdef DEBUG
+#if defined(DEBUG) && FDBGL & D_LOGS_ITER_BLOCK
 static inline void
 printf_ban_times(const uint8_t *banspec, unsigned len, const char *fmt)
 {