selftests/bpf: Whitelist test_progs.h from .gitignore

Somehow test_progs.h was being included by the existing rule:

    /test_progs*

This is bad because:

    1) test_progs.h is a checked in file
    2) grep-like tools like ripgrep[0] respect gitignore and
       test_progs.h was being hidden from searches

[0]: https://github.com/BurntSushi/ripgrep



Fixes: 74b5a596 ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/a46f64944bf678bc652410ca6028d3450f4f7f4b.1623880296.git.dxu@dxuuu.xyz