Revert "ltp-testsuite: don't link with libtirpc"

This reverts commit 8f1c4c49 in favor of an
alternative solution in a subsequent patch.

Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>