Fix incorrect function call (#3232)

php/src/Google/Protobuf/Internal/Descriptor.php

@@ -129,7 +129,7 @@ class Descriptor
         $message_name_without_package  = "";
         $classname = "";
         $fullname = "";
-        getFullClassName(
+        GPBUtil::getFullClassName(
             $proto,
             $containing,
             $file_proto,