selftests/bpf: fix test_verifier/test_maps make dependencies

e46fc22e ("selftests/bpf: make directory prerequisites order-only")
exposed existing problem in Makefile for test_verifier and test_maps tests:
their dependency on auto-generated header file with a list of all tests wasn't
recorded explicitly. This patch fixes these issues.

Fixes: 51a0e301 ("bpf: Add BPF_MAP_TYPE_SK_STORAGE test to test_maps")
Fixes: 6b7b6995 ("selftests: bpf: tests.h should depend on .c files, not the output")
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>