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

lib/formdata.c

@@ -342,6 +342,7 @@ CURLFORMcode FormAdd(struct curl_httppost **httppost,
 #else
       current_form->flags |= HTTPPOST_PTRNAME; /* fall through */
 #endif
+      /* FALLTHROUGH */
     case CURLFORM_COPYNAME:
       if(current_form->name)
         return_value = CURL_FORMADD_OPTION_TWICE;

lib/ftp.c

@@ -3214,6 +3214,7 @@ static CURLcode ftp_done(struct connectdata *conn, CURLcode status,
 
     /* until we cope better with prematurely ended requests, let them
      * fallback as if in complete failure */
+    /* FALLTHROUGH */
   default:       /* by default, an error means the control connection is
                     wedged and should not be used anymore */
     ftpc->ctl_valid = FALSE;

lib/vtls/openssl.c

@@ -539,6 +539,7 @@ int cert_stuff(struct connectdata *conn,
       if(!key_file)
         /* cert & key can only be in PEM case in the same file */
         key_file=cert_file;
+      /* FALLTHROUGH */
     case SSL_FILETYPE_ASN1:
       if(SSL_CTX_use_PrivateKey_file(ctx, key_file, file_type) != 1) {
         failf(data, "unable to set private key file: '%s' type %s",

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;