mkqt5bld: Add a global definition for missing variables

 W:1001,4:parse_cmd_line: Global variable 'QTCREATOR_VERSION' undefined at the module level
 W:1002,4:parse_cmd_line: Global variable 'QTCREATOR_VERSION_DESCRIPTION' undefined at the module level
 W:1054,4:setup_option_parser: Global variable 'OPTION_PARSER' undefined at the module level

Change-Id: I169021ec04e82935f059d4f9083027210e655df3
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>