[PATCH bpf] selftests: bpf: fix makefile dependencies on libbpf

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

quoted

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 3ac2e20fba07e 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: 3ac2e20fba07e ("selftests/bpf: BPF object files should depend only on libbpf headers")
Signed-off-by: Jiri Benc <redacted>
---
 tools/testing/selftests/bpf/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 866531c08e4f..e7c42695dbbf 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/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)