build_wrapper: Fix typo

   File "../qtsdk/release-tools/build_wrapper.py", line 238, in handle_qt_src_package_build
     cmd_args = [os.path.join(SCRIPT_ROOT_DIR, + 'mksrc.sh'), '-u', os.path.join(WORK_DIR, 'qt5')]
 TypeError: bad operand type for unary +: 'str'

Change-Id: I1b1a53067b196b60fbe08821112fb2afd228aaf5
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>