qlnx: gcc build errors

Propagate warning flags from kern.opts.mk and then fix minor -Werror
issues when building with gcc from -Wredundant-decls, -Wnested-externs,
-Wuninitialized.

Reviewed by: davidcs
Approved by: markj (mentor)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11413