Make windows.h lower case since it is the correct filename

[Viatorus]

Windows itself is case insensitve, but the correct filename of windows.h is written in lower case.

If the lv_drivers is built cross-compiled for Windows from Linux via MinGW, this leads to an error, since Linux is case sensitive.

See: https://stackoverflow.com/questions/15466613/lowercase-windows-h-and-uppercase-windows-h-difference

[kisvegabor]

Thank you!

[MouriNaruto]

@Viatorus

In Windows SDK, the file name is Windows.h. So, Windows.h is not the wrong file name. (It's the MinGW's issue.)

But the change is good for doing workaround for MinGW.

Kenji Mouri