libtirpc: fix compilation error of rpcgen

When compiling libtirpc, without RPC support available on the host
machine, the build of the rpcgen host program because it cannot find
the netconfig.h and rpc/types.h headers. Instead of relying on the
system-provided ones, let's use the ones included in the libtirpc
source code by patching the rpcgen build logic.

Signed-off-by: Dagg Stompler <daggs@gmx.com>
[Thomas: reword commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>