kern.mk: fix -Wno-error style to fix build with Clang 12

Clang 12 no longer supports -Wno-error-..., only the -Wno-error=...
style (which is already used everywhere else in the tree).

Differential Revision: https://reviews.freebsd.org/D29157

(cherry picked from commit 15565e0a2177f53b086609fecd48991c52dad5eb)
(cherry picked from commit 742d315019789de01d5dbf736cbafd49cc603276)