[lld-macho] Fix BUILD_SHARED_LIBS build

ca675104 added a dependency on XAR (at
least for the shared libs build), so without this change we get the
following linker error:

Undefined symbols for architecture x86_64:
  "_xar_close", referenced from:
      lld::macho::BitcodeBundleSection::finalize() in SyntheticSections.cpp.o

Reviewed By: #lld-macho, int3, thakis

Differential Revision: https://reviews.llvm.org/D100999