PHP 7.3 support for fix_packages

[sbulen]

Class type declarations were added in PHP 7.4. In theory we still support older versions of PHP...

So... In PHP 7.3, this throws an error:

tools/fix_packages.php

Line 90 in 4cd83c2

private bool $isInstalling;

PHP Parse error: syntax error, unexpected 'bool' (T_STRING), expecting function (T_FUNCTION) or const (T_CONST) in /var/www/html/forum/fix_packages.php on line 90

I believe this is the only tweak needed to support 7.3.

This was reported in the forum:
https://www.simplemachines.org/community/index.php?topic=588937.0