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.

Change-Id: Ida1608774de3bc5861b223574f3cbf0d9647266c
Reviewed-on: http://gerrit.openafs.org/3682
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>