windployqt: Fix cross-compilation.

Fix signature of readPeExecutable().

Task-number: QTBUG-38160
Task-number: QTBUG-38886
Change-Id: I3ff88d502e0f9f7912b4c0741667a8f4ff71dbba
Reviewed-by: Kai Koehne <kai.koehne@digia.com>