resource_bundle_mac_unittest.mm: fix case for AppKit/AppKit.h
resource_bundle_mac_unittest.mm: fix case for AppKit/AppKit.h

in SDK, it is AppKit.framework/Versions/C/Headers/AppKit.h.
Use the same case, so it would work on case sensitive filesystem.

BUG=

Review URL: https://codereview.chromium.org/1454113004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#361259}

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