Fix include path for (libdrm/)drm_fourcc.h

This PR broke a private nixpkgs definition I have for wlroots: https://github.com/swaywm/wlroots/pull/1304

It is fixed by changing #include <drm_fourcc.h> to #include <libdrm/drm_fourcc.h>, which follows what is already done in the dmabuf example.