Fix msvc warning: 'inline' : macro redefinition

Fixes msvc compiler warnings:
```
d:\build\pillow\pillow-git\libimaging\ImPlatform.h(25) : warning C4005: 'inline' : macro redefinition
        d:\build\pillow\pillow-git\libimaging\ImPlatform.h(21) : see previous definition of 'inline'
```