ftp: fix build for CURL_DISABLE_VERBOSE_STRINGS

This is a follow-up to b7c7dffe which changed the FTP state change
verbose debug text (aka infof) to tracing debug text (aka trc).

Prior to this change if libcurl was without DEBUGBUILD and built with
CURL_DISABLE_VERBOSE_STRINGS (ie --disable-verbose) the build would
error.

Caught by Circle CI job openssl-no-verbose.

lib/ftp.c

@@ -154,7 +154,9 @@ static void _ftp_state(struct Curl_easy *data,
   struct ftp_conn *ftpc = &conn->proto.ftpc;
 
 #if defined(CURL_DISABLE_VERBOSE_STRINGS)
+#ifdef DEBUGBUILD
   (void)lineno;
+#endif
 #else /* CURL_DISABLE_VERBOSE_STRINGS */
   if(ftpc->state != newstate)
 #ifdef DEBUGBUILD