selftests/bpf: prevent headers to be compiled as C code

Apprently listing header as a normal dependency for a binary output
makes it go through compilation as if it was C code. This currently
works without a problem, but in subsequent commits causes problems for
differently generated test.h for test_progs. Marking those headers as
order-only dependency solves the issue.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>