build: fix gcc7 implicit fallthrough warnings

Mark intended fallthroughs with /* FALLTHROUGH */ so that gcc will know
it's expected and won't warn on [-Wimplicit-fallthrough=].

Closes https://github.com/curl/curl/pull/1297

src/tool_cb_dbg.c

@@ -185,6 +185,7 @@ int tool_debug_cb(CURL *handle, curl_infotype type,
   switch(type) {
   case CURLINFO_TEXT:
     fprintf(output, "%s== Info: %s", timebuf, data);
+    /* FALLTHROUGH */
   default: /* in case a new one is introduced to shock us */
     return 0;