Using a relative path for the pthread.h header by w32-pthreads breaks
compilation of plugins which include threading.h from libobs (as
w32-pthreads will exist at a different location relative to the
libobs header).
As w32-pthreads (and its include directory) is added to the libobs
target by CMake, the pthread.h header will be found even when using
system header notation.
Fixes https://github.com/obsproject/obs-studio/issues/7155