Fix UWP builds

[Osyotr]

UWP does not support memory mapped files.

Found while updating version in vcpkg: microsoft/vcpkg#32074

[syoyo]

Oh its interesting that UWP doesn't support mmap.

Considering other Windows build configuration(e.g. mingw), it'd be better to write something like:

#if !defined(UWP_BUILD) 
#define TINYEXR_USE_WIN32_MMAP (1)
#endif

to disable mmap only for UWP build.

What macro will be defined in UWP build?

[Osyotr]

I think WINAPI_FAMILY is the right define to check for UWP. I've updated the condition to only trigger when it's defined, without introducing new define in global scope.

[syoyo]

Thanks! Merged!