Export prototypes for osi_fbsd_{alloc,free} for use in rx

Export prototypes for osi_fbsd_{alloc,free} for use in rx

Include prototypes for osi_fbsd_alloc() and osi_fbsd_free() in
osi_machdep.h, since afs_prototypes.h is not included when compiling
the rx code.  afs_osi_Alloc_NoSleep is #defined to be osi_fbsd_free,
and is used in the rx code; if the prototype for the latter is not
included, then int is assumed for all parameters and return values,
which breaks the calling convention for 64-bit pointers.

Change-Id: I3d7575525f159097d1f8ca9ecb22af9f27b63111
Reviewed-on: http://gerrit.openafs.org/1541
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>