configurejson2cmake: Follow up fix to a regression

UnboundLocalError: local variable
 'skip_special_case_preservation' referenced before assignment

Change-Id: Ic7419e0bdd04a4251182d1a2616cd57e89e62432
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

util/cmake/configurejson2cmake.py

@@ -1562,6 +1562,7 @@ def main():
         print("This scripts needs one directory to process!")
         quit(1)
 
+    skip_special_case_preservation = False
     if len(sys.argv) > 2 and sys.argv[2] == '-s':
         skip_special_case_preservation = True