Wrap xar/xar.h include in extern "C" block

Without such wrapping, linking lld fails with missing symbols because of
C++ symbol mangling with older versions of the MacOSX SDK, in which
xar.h doesn't have an extern "C" block itself.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D110224