openzfs: fix gcc kernel module builds

[rlibby]

Summary

• Suppress -Wredundant-decls. Ultimately this warning is harmless in any case, and it does not look like there is a simple way to avoid redundant declarations in this case without a lot of header pollution (e.g. having openzfs's shim param.h pulling in sys/kernel.h for hz).
• Suppress -Wnested-externs, which is useless anyway.

Unfortunately it was not sufficient just to modify OPENZFS_CFLAGS,
because the warning suppressions need to appear on the command line
after they are explicitly enabled by CWARNFLAGS from sys/conf/kern.mk,
but OPENZFS_CFLAGS get added before due to use of -I for the shims.

Test Plan

make buildkernel

env MAKEOBJDIRPREFIX=/usr/obj/gcc9 CROSS_TOOLCHAIN=amd64-gcc9 make buildkernel

[rlibby]

@gnn did you want to take a look at this patch before it goes in? It is just turning down nuisance warnings for gcc compatibility.