selftests/vm: fix link error for transhuge-stress test

add -lrt to fix undefined reference to `clock_gettime'
error seen when the test is compiled using gcc 4.6.4.

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>