package/libtirpc: Do not build unsupported API files

Version 0.3.2 re-enables some APIs for backward compatibility with
glibc's SunRPC:
  - key_prot
  - netname
  - rtime

Unfortunalty, this breaks building libtirpc with uClibc and musl:
  getpublickey.c:41:28: fatal error: rpcsvc/yp_prot.h: No such file or directory

We disable those API files as at least yp_prot.h is also NIS related to support
building libtirpc with these two libc's.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>