libbpf: Ignore hashmap__find() result explicitly in btf_dump

Coverity is reporting that btf_dump_name_dups() doesn't check return
result of hashmap__find() call. This is intentional, so make it explicit
with (void) cast.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221117192824.4093553-1-andrii@kernel.org