xen: update pvcalls_front_accept prototype

While currently there are no in-tree callers of these functions, it is
best to keep them up-to-date with the latest network API.

In addition, add the missing EXPORT_SYMBOL_GPL to the functions listed
in pvcalls-front.h.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <alpine.DEB.2.22.394.2501201537560.11086@ubuntu-linux-20-04-desktop>
Signed-off-by: Juergen Gross <jgross@suse.com>