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>

qmake/library/qmakebuiltins.cpp

@@ -195,7 +195,7 @@ static bool isTrue(const ProString &_str, QString &tmp)
     return !str.compare(statics.strtrue, Qt::CaseInsensitive) || str.toInt();
 }
 
-#ifdef Q_OS_WIN
+#if defined(Q_OS_WIN) && defined(PROEVALUATOR_FULL)
 static QString windowsErrorCode()
 {
     wchar_t *string = 0;