kcov: fix build on aarch64

Apparently, without this patch `NT_PRSTATUS` is not found. So the patch
adds the include apparently necessary. `NT_PRSTATUS` is also defined in
`<linux/ptrace.h>`, which would likely have been a better name, were it
not in the `linux/` directory, which is a priori not stable.

The need to do that is kind of weird (the change was introduced in [1],
and fedora apparently didn't need this additional import), but I'll try
to upstream it.

[1] https://github.com/SimonKagstrom/kcov/pull/239