infiniband: Disable -Wredundant-decl warnings.

ib_uverbs_flow_resources_free() is declard in two header files in
upstream OFED. Disable the warning to avoid introducing diffs to fix
the build on GCC 9.

While here, fix the ibcore module to disable the same warnings
disabled in OFED_CFLAGS.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D31943

(cherry picked from commit 44126818d2ec96d8247eafcae9b96905a473a264)