selftests/bpf: Fix dependency ordering for attach_probe test

Current Makefile dependency chain is not strict enough and allows
test_attach_probe.o to be built before test_progs's
prog_test/attach_probe.o is built, which leads to assembler complaining
about missing included binary.

This patch is a minimal fix to fix this issue by enforcing that
test_attach_probe.o (BPF object file) is built before
prog_tests/attach_probe.c is attempted to be compiled.

Fixes: 928ca75e

 ("selftests/bpf: switch tests to new bpf_object__open_{file, mem}() APIs")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191007204149.1575990-1-andriin@fb.com