vmgenc: fix typo in MODULE_DEPEND declaration

The random_harvestq dependency was registered under the misspelled
name "vemgenc" instead of "vmgenc", causing the dependency to not
be associated with the correct module.

Signed-off-by: Christos Longros <chris.longros@gmail.com>
Reviewed by: cem, imp
Differential Revision: https://reviews.freebsd.org/D56012