Export header file

[Explosiontime202]

I am currently trying to include the project into my CMake project using FetchContent. So far, so good. The issue is that the header files are not publicly exported in your CMakeLists.txt.

Added the following line in line 84 would solve the issue:

set_target_properties(osqp-cpp PROPERTIES PUBLIC_HEADER "include/osqp++.h")

I can also provide a PR.