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: NYonghong Song <yhs@fb.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NJiong Wang <jiong.wang@netronome.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/.gitignore

@@ -29,3 +29,4 @@ test_netcnt
 test_section_names
 test_tcpnotify_user
 test_libbpf
+alu32