selftests/bpf: Fix .gitignore to not ignore test_progs.c

List all possible test_progs flavors explicitly to avoid accidentally
ignoring valid source code files. In this case, test_progs.c was still
ignored after recent 809ed84d ("selftests/bpf: Whitelist test_progs.h
from .gitignore") fix that added exception only for test_progs.h.

Fixes: 74b5a596

 ("selftests/bpf: Replace test_progs and test_maps w/ general rule")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210914162228.3995740-1-andrii@kernel.org