Guard call to "get_magic_quotes_gpc()" with "@" to silence PHP 7.4+ warning

Summary:
Fixes T13471. Recent versions of PHP raise a warning when this function is called.

We're only calling it so we can instantly fatal if it's enabled, so use "@" to silence the warning.

Test Plan: Loaded site; see also T13471 for a user reporting that this fix is effective.

Maniphest Tasks: T13471

Differential Revision: https://secure.phabricator.com/D20942