Fix PhutilLibraryMapBuilder to call the right function in log()

Summary:
`arc liberate` has been broken for a while because the `log` function attempts to call `fwrite` (whose third argument is an integer length) instead of `fprintf`. I think maybe PHP 5 was more lenient about this? Anyway, this bug makes other development somewhat tricky.

The arcanist source in general seems to be split between `fwrite(STDERR, $message."\n")` and `fprintf(STDERR, "%\n", $message)`; I decided to go with the latter.

This is sort of a test balloon on submitting diffs to Phorge. Please feel free to let me know if there's anything you'd like done differently!

Test Plan: Ran `arc liberate` to regenerate source maps and didn't get any errors

Reviewers: O1 Blessed Committers, eax

Reviewed By: O1 Blessed Committers, eax

Subscribers: chris, speck, tobiaswiese

Differential Revision: https://we.phorge.it/D25017

src/moduleutils/PhutilLibraryMapBuilder.php

@@ -122,7 +122,7 @@ final class PhutilLibraryMapBuilder extends Phobject {
    */
   private function log($message) {
     if (!$this->quiet) {
-      @fwrite(STDERR, "%s\n", $message);
+      @fprintf(STDERR, "%s\n", $message);
     }
     return $this;
   }