random/jitterentropy: fix USE_JENT == JENT_USES_GETTIME code path

* random/jitterentropy-base-user.h (jent_get_nstime): Use 'tv' variable
instead of non-existing 'time'.

Backported-from-master: 4ed9b949485448816a70d86260d572f08ae34621

Note that on any decent x86 hardware that code path is not used.

• GnuPG-bug-id: T4966
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>