Fix warning

Warnings came up using MinGW 4.4:
* qmakebuiltins.cpp: 'QString windowsErrorCode()' defined but not used
[...]

Change-Id: I80827f7f740ff380f13cef96d6b43baaa7e179cb
(cherry picked from qtcreator/0e13cf3faffc7463ae2bf535dfdb008202112cc4)
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>