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

tools/testing/selftests/bpf/Makefile

@@ -164,7 +164,7 @@ $(OUTPUT)/test_queue_map.o: test_queue_stack_map.h
 $(OUTPUT)/test_stack_map.o: test_queue_stack_map.h
 
 $(OUTPUT)/flow_dissector_load.o: flow_dissector_load.h
-$(OUTPUT)/test_progs.o: flow_dissector_load.h $(OUTPUT)/test_attach_probe.o
+$(OUTPUT)/test_progs.o: flow_dissector_load.h
 
 BTF_LLC_PROBE := $(shell $(LLC) -march=bpf -mattr=help 2>&1 | grep dwarfris)
 BTF_PAHOLE_PROBE := $(shell $(BTF_PAHOLE) --help 2>&1 | grep BTF)
@@ -275,7 +275,7 @@ PROG_TESTS_H := $(PROG_TESTS_DIR)/tests.h
 PROG_TESTS_FILES := $(wildcard prog_tests/*.c)
 test_progs.c: $(PROG_TESTS_H)
 $(OUTPUT)/test_progs: CFLAGS += $(TEST_PROGS_CFLAGS)
-$(OUTPUT)/test_progs: test_progs.c $(PROG_TESTS_FILES) | $(PROG_TESTS_H)
+$(OUTPUT)/test_progs: test_progs.c $(PROG_TESTS_FILES) | $(OUTPUT)/test_attach_probe.o $(PROG_TESTS_H)
 $(PROG_TESTS_H): $(PROG_TESTS_FILES) | $(PROG_TESTS_DIR)
 	$(shell ( cd prog_tests/; \
 		  echo '/* Generated header, do not edit */'; \