Add property type declarations

Property type declarations for class variables were added in PHP
7.4.

MDErrorReporter.php

@@ -14,9 +14,9 @@ final class MDErrorReporter {
     const MD_ERROR_KNOWN    = 0;
 
     /** @var string */
-    private $_context;
+    private string $_context;
     /** @var string */
-    private $_contact_mail_addr;
+    private string $_contact_mail_addr;
 
     /**
      * Categorizes an error to better decide how to act on it.