Add missing expansion for $$_PRO_FILE_PWD_

Tests in qtdeclarative were failing because the above variable was
expanded to an empty string. This causes the tests to be unable to
locate their test data when executed.

Change-Id: Ibc3c094123f25d688a73c11886ac1673b6930c54
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

util/cmake/pro2cmake.py

@@ -594,7 +594,7 @@ class Scope(object):
 
         is_same_path = self.currentdir == self.basedir
 
-        if key == 'PWD':
+        if key == '_PRO_FILE_PWD_' or key == 'PWD':
             if is_same_path:
                 return ['${CMAKE_CURRENT_SOURCE_DIR}']
             else: