MFC r320152 (by avg): fstyp: move sys/ include path after zfs include paths

The reason is that FreeBSD refcount.h shadows ZFS refcount.h and that
will lead to a build error after a planned import of the ARC buf data
scatter-ization.
It's possible that some day we will have an opposite problem where
a ZFS header would shadow an essential FreeBSD header.
So, we need to think about a better long term solution.