OpenBSD: Fix use of macros for AFS_KALLOC/AFS_KFREE

OpenBSD: Fix use of macros for AFS_KALLOC/AFS_KFREE

The macro definitions for AFS_KALLOC and AFS_KFREE used by
afs_osi_alloc.c to perform OS-specific memory allocation
are only defined by the OpenBSD osi_machdep.h header file
if they already exist, which means: never. This patch fixes
this situtation and makes sure that afs_osi_alloc.c
actually uses them, eliminating a direct OS-specific call
in afs_osi_alloc.c.

Reviewed-on: http://gerrit.openafs.org/3682
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit d5367b5f1494ca2cf09f62d7f2ffa6304e38a648)

Change-Id: I362f1b6ab808ff171cebc429eb74703c7151d026
Reviewed-on: http://gerrit.openafs.org/3718
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>