compat: Remove non-Mac dependency on third_party/xnu
compat: Remove non-Mac dependency on third_party/xnu

It looks like this was planned for future changes that haven't happened
yet. Current users of the .def files explicitly reference them, so
compat currently doesn't require this dependency.

Bug: fuchsia:66116
Change-Id: I85404d17eac4a2aa3c2157fe3fc2a1421879996a
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2640725
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Scott Graham <e134406e1863f3b48b0795831afac5c0595efb36@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad