Fix build without libunwind installed (#17221)

Fix build without libunwind installed (#17221)

* Fix build without libunwind installed

I have removed libunwind dependency in a recent commit, but
it turns out that the build was using incorrect include
paths and it was still depending on the libunwind installation
for the libunwind.h header.

This change fixes it.

* Fix clang warning

The libunwind headers contain two empty structs and clang
issues a warning since the size of such structs is
0 in C and 1 in C++.
This is benign for our purposes, since the affected
structs are not directly used by our code.