Fix msvc warning: 'inline' : macro redefinition

[cgohlke]

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'

[coveralls]

Coverage remained the same when pulling b593ff0 on cgohlke:patch-2 into 68c502d on python-pillow:master.

[hugovk]

Thanks.