MDL-77350 filter: Added class properties that are not declared

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

filter/tex/latex.php

@@ -10,6 +10,9 @@
         var $temp_dir;
         var $error;
 
+        /** @var bool To store value of supported_platform. */
+        protected $supported_platform;
+
         /**
          * Constructor - create temporary directories and build paths to
          * external 'helper' binaries.