selftests/bpf: Fix .gitignore to ignore no_alu32/

When switching to alu32 by default, no_alu32/ subdirectory wasn't added
to .gitignore. Fix it.

Fixes: e13a2fe6

 ("tools/bpf: Turn on llvm alu32 attribute by default")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20191025045503.3043427-1-andriin@fb.com