tools/crypto_bench: fix build when AV_READ_TIME is unavailable

Found-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 4a99134f1a71994a0dc4542a0d6bee8e36146b60)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>