net/nfstrace: Fix build with json-c 0.19

json-c 0.19 added static inline accessors to <json-c/linkhash.h>,
which trigger -Wunused-function when building the bundled JSON
analyzer with -Werror.

Suppress the warning by adding -Wno-unused-function to CFLAGS.

Reported by: olgeni
Differential Revision: https://reviews.freebsd.org/D58135