Skip anon functions in symbol generation script

Summary:
Filters closures out of symbol generator script, per @epriestley's
comment in T4334

Test Plan:
Before:
eric@eric-dev ~/phabricator/scripts/symbols: echo 'closure.php' | ./generate_php_symbols.php
function php /closure.php
d function php 10 /closure.php
function php /closure.php
a class php 3 /closure.php
a b method php 4 /closure.php

After:
eric@eric-dev ~/phabricator/scripts/symbols: echo 'closure.php' | ./generate_php_symbols.php
d function php 10 /closure.php
a class php 3 /closure.php
a b method php 4 /closure.php

eric@eric-dev ~/phabricator/scripts/symbols: cat closure.php
<?php

class a {
function b() {
$c = function() { return 1; };
$c();
}
}

function d() {
return 2;
}
$e = function() {
return 3;
};

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: epriestley, Korvin, aran

Differential Revision: https://secure.phabricator.com/D8054