[libunwind] Install the headers by default

Just like we install libc++ and libc++abi headers by default when we
install the library, it makes sense to install the libunwind headers
by default when we build libunwind. In the current state of things,
there is an increased risk that folks are using older (previously
installed) libunwind headers along with a recent libunwind dylib,
which is not ideal.

Differential Revision: https://reviews.llvm.org/D135663