Fix previous commit (do not print warning even for wrong passphrase).
Link against -lrt for clock_gettime

Fix undefined reference to `clock_gettime' error.
This is required since commit f3e398afc549 "Rewrite cipher benchmark loop.".

Below is the build error report:
http://autobuild.buildroot.net/results/9ca/9cad1cf49ee6b5c58d8260ee33beef2e6c1ada4d/build-end.log

Signed-off-by: Axel Lin <axel.lin@ingics.com>