rxgen: Only cast array/pointer/vector types

Assuming the correct values are passed to the xdr functions, no casts
are required. Don't cast simple/struct/union/typedef values. Do cast
array/pointer/vectors, since the relevant xdr wrapper functions expect
char *.

Change-Id: I375c03899576735668c1a0df0af47377223ae97a
Reviewed-on: http://gerrit.openafs.org/10260
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>