selftests: bpf: tests.h should depend on .c files, not the output

This makes sure we don't put headers as input files when doing
compilation, because clang complains about the following:

clang-9: error: cannot specify -o when generating multiple output files
../lib.mk:152: recipe for target 'xxx/tools/testing/selftests/bpf/test_verifier' failed
make: *** [xxx/tools/testing/selftests/bpf/test_verifier] Error 1
make: *** Waiting for unfinished jobs....
clang-9: error: cannot specify -o when generating multiple output files
../lib.mk:152: recipe for target 'xxx/tools/testing/selftests/bpf/test_progs' failed

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>