Fix signed/unsigned warnings in XDR

Fix signed/unsigned warnings in XDR

Our xdr routines use the same native functions to read signed, and
unsigned integers from the wire. This leads to compiler warnings when
the unsigned versions of these functions are called. This patch always
casts to (afs_int32 *) when calling PUT_INT32 and GET_INT32, to resolve
these warnings

Reviewed-on: http://gerrit.openafs.org/507
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>