libs/gnttab: add missing FreeBSD functions

libs/gnttab: add missing FreeBSD functions

The FreeBSD implementation is missing the following functions:

osdep_gnttab_dmabuf_exp_from_refs
osdep_gnttab_dmabuf_exp_wait_released
osdep_gnttab_dmabuf_imp_to_refs
osdep_gnttab_dmabuf_imp_release

Which all deal with dmabufs, that only exists on Linux. Implement them
using abort, since such functions should never be called on FreeBSD.

FTR, I realized those functions where missing when attempting to use
pygrub:

Traceback (most recent call last):
  File "/usr/local/lib/xen/bin/pygrub", line 19, in <module>
    import xen.lowlevel.xc
ImportError: /usr/local/lib/libxengnttab.so.1: Undefined symbol "osdep_gnttab_dmabuf_exp_from_refs"

Fixes: ee8105 ("libgnttab: Add support for Linux dma-buf")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>