mkqt5bld: Add a global definition for QT_CREATOR_SRC_DIR

  W:751,4:parse_cmd_line: Global variable 'QT_CREATOR_SRC_DIR' undefined at the module level

This seems to be a left over from 73821377c883fcf80159b11802fec331f0dd0716

Change-Id: I3bc5e8c7c44e043ca24fb68b62ec147f325496fc
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>