Add headers needed by pg_combinebackup --clone

Add headers needed by pg_combinebackup --clone

The code for file cloning existed, but was not reachable as it relied on
constants from missing headers. Due to that, on Linux --clone always
failed with

  error: file cloning not supported on this platform

Fixed by including the missing headers to relevant places. Adding the
headers revealed a couple compile errors in copy_file_clone(), so fix
those too.

Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/48da4a1f-ccd9-4988-9622-24f37b1de2b4%40eisentraut.org