ltp-testsuite: don't link with libtirpc

Buildroot patches libtirpc to remove auth_des support. This breaks tirpc test
build. Remove support for libtirpc for now.

Fixes:
http://autobuild.buildroot.net/results/301/3015eee7b1b6b240e7948b08954d273d28f44c32/



Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>