Fix ArcanistFormattedStringXHPASTLinterRule on older PHP after D21500

Summary:
Calling 'Foo::bar' is only supported since PHP 7, whereas the array form
is supported since PHP 5.4, which is below our PHP 5.5 baseline.

Test Plan: No regressions under PHP 8 and snippet tested on 3v4l

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21505

src/lint/linter/xhpast/rules/ArcanistFormattedStringXHPASTLinterRule.php

@@ -94,10 +94,7 @@ final class ArcanistFormattedStringXHPASTLinterRule
       $argv = array($format->evalStatic()) + array_fill(0, $argc, null);
 
       try {
-        xsprintf(
-          'ArcanistFormattedStringXHPASTLinterRule::processXsprintfCallback',
-          null,
-          $argv);
+        xsprintf(array(__CLASS__, 'processXsprintfCallback'), null, $argv);
       } catch (BadFunctionCallException $ex) {
         $this->raiseLintAtNode(
           $call,