btrfs-progs: convert: drop dependency on sys/acl.h

btrfs-progs: convert: drop dependency on sys/acl.h

The file sys/acl.h is part of libacl and if the development package is
not installed, build of btrfs-convert fails.

We do not link against libacl nor use the functions provided by libacl.  The
ACL_* values are directly read from the extN data, so it's more part of the
on-disk format rather than an interface to libacl.

The dependency on libacl is completely dropped.

Reported-by: Hugo Mills <hugo@carfax.org.uk>
Signed-off-by: David Sterba <dsterba@suse.cz>