This is missing, and is defined [here](https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/bits/sched.h;h=b27755c39eeed365ec249638225713971f1367da;hb=HEAD#l37) in glibc and [here](https://git.musl-libc.org/cgit/musl/tree/include/sched.h?id=1e21e78bf7a5c24c217446d8760be7b7188711c2) in musl. This really is Linux-only.
