Incomplete include dir spec

[tschiemer]

If incorporating oscpack through cmake add_subdirectory compilation will fail because headers are not found.

This is because the include directory does not reference the current directory, ie CMAKE_CURRENT_SOURCE_DIR.

https://github.com/MariadeAnton/oscpack/blob/1cb90c372a182ac13b9a5f68b938c06ade851836/CMakeLists.txt#L26

Also consider using target_include_directories(oscpack PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}) to directly export the directory to any including projects without having to care much about include paths therein.