openzfs: fix gcc in-kernel builds

Fix gcc builds of in-kernel modules using CDDL_CFLAGS (e.g. options
ZFS).

• Remove nonexistent include dirs for -Wmissing-include-dirs
• Suppress -Wnested-externs, a useless warning
• Sort and uniq warning flags while here

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27684