selftests/bpf: Silence unused-but-set build warnings

There are some set but not used build errors when compiling bpf selftests
with the latest upstream mainline GCC, at the beginning add the attribute
__maybe_unused for the variables, but it is better to just add the option
-Wno-unused-but-set-variable to CFLAGS in Makefile to disable the errors
instead of hacking the tests.

tools/testing/selftests/bpf/map_tests/lpm_trie_map_basic_ops.c:229:36:
error: variable ‘n_matches_after_delete’ set but not used [-Werror=unused-but-set-variable=]

tools/testing/selftests/bpf/map_tests/lpm_trie_map_basic_ops.c:229:25:
error: variable ‘n_matches’ set but not used [-Werror=unused-but-set-variable=]

tools/testing/selftests/bpf/prog_tests/bpf_cookie.c:426:22:
error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

tools/testing/selftests/bpf/prog_tests/find_vma.c:52:22:
error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

tools/testing/selftests/bpf/prog_tests/perf_branches.c:67:22:
error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

tools/testing/selftests/bpf/prog_tests/perf_link.c:15:22:
error: variable ‘j’ set but not used [-Werror=unused-but-set-variable=]

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/20251018082815.20622-1-yangtiezhu@loongson.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>