Linux mingw uses lowercase headers, so cross-building on Linux for Windows
target must use the correct form. Windows is case-insensitive, so it won't
affect host-native builds.
Change-Id: Ic2140412bb367bbce83b5e2546d70e0936928c5c
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: David Schulz <david.schulz@qt.io>