Tell `FindOSL.cmake` where to find the shader header files. These have
moved from `${LIBDIR}/osl/shaders` to `${LIBDIR}/osl/share/OSL/shaders`
when OSL was upgraded (T85365).
Tell `FindOSL.cmake` where to find the shader header files. These have
moved from `${LIBDIR}/osl/shaders` to `${LIBDIR}/osl/share/OSL/shaders`
when OSL was upgraded (T85365).