selftest/bpf/benchs: Make a function static in bpf_hashmap_full_update

The hashmap_report_final callback function defined in the
benchs/bench_bpf_hashmap_full_update.c file should be static.

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230213091519.1202813-3-aspsk@isovalent.com