selftests/bpf: Mark variable as static

Fix warnings from checkstyle.pl

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211112192535.898352-4-fallentree@fb.com