libtirpc: rpcgen is only built for host, don't install to target

rpcgen is only used to generate headers needed for compilation.
Move it to noinst_PROGRAMS otherwise you may end up with a host-arch
binary in your target.

Signed-off-by: Phil Eichinger <phil@zankapfel.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>