selftests: bpf: Fix makefile dependencies on libbpf

When building bpf selftest with make -j, I'm randomly getting build failures
such as this one:

  In file included from progs/bpf_flow.c:19:
  [...]/tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:11:10: fatal error: 'bpf_helper_defs.h' file not found
  #include "bpf_helper_defs.h"
           ^~~~~~~~~~~~~~~~~~~

The file that fails the build varies between runs but it's always in the
progs/ subdir.

The reason is a missing make dependency on libbpf for the .o files in
progs/. There was a dependency before commit 3ac2e20 but that commit
removed it to prevent unneeded rebuilds. However, that only works if libbpf
has been built already; the 'wildcard' prerequisite does not trigger when
there's no bpf_helper_defs.h generated yet.

Keep the libbpf as an order-only prerequisite to satisfy both goals. It is
always built before the progs/ objects but it does not trigger unnecessary
rebuilds by itself.

Fixes: 3ac2e20 ("selftests/bpf: BPF object files should depend only on libbpf headers")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/ee84ab66436fba05a197f952af23c98d90eb6243.1632758415.git.jbenc@redhat.com

tools/testing/selftests/bpf/Makefile

@@ -375,7 +375,8 @@ $(TRUNNER_BPF_OBJS): $(TRUNNER_OUTPUT)/%.o:				\
 		     $(TRUNNER_BPF_PROGS_DIR)/%.c			\
 		     $(TRUNNER_BPF_PROGS_DIR)/*.h			\
 		     $$(INCLUDE_DIR)/vmlinux.h				\
-		     $(wildcard $(BPFDIR)/bpf_*.h) | $(TRUNNER_OUTPUT)
+		     $(wildcard $(BPFDIR)/bpf_*.h)			\
+		     | $(TRUNNER_OUTPUT) $$(BPFOBJ)
 	$$(call $(TRUNNER_BPF_BUILD_RULE),$$<,$$@,			\
 					  $(TRUNNER_BPF_CFLAGS))