selftests: bpf: add "alu32" to .gitignore

"alu32" is a build dir and contains various files for BPF sub-register
code-gen testing.

This patch tells git to ignore it.

Suggested-by: Yonghong Song <yhs@fb.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>