xdrgen: Fix struct prefix for typedef types in program wrappers

xdrgen: Fix struct prefix for typedef types in program wrappers

The program templates for decoder/argument.j2 and encoder/result.j2
unconditionally add 'struct' prefix to all types. This is incorrect
when an RPC protocol specification lists a typedef'd basic type or
an enum as a procedure argument or result (e.g., NFSv2's fhandle or
stat), resulting in compiler errors when building generated C code.

Fixes: 4b132aacb076 ("tools: Add xdrgen")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>