bpf: Rebuild bpf.o for any dependency update

This is needed to force a rebuild of bpf.o when one of its dependencies
(e.g. uapi/linux/bpf.h) is updated.

Add a phony target.
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Wang Nan <wangnan0@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/Makefile

-CFLAGS += -Wall -O2 -lcap -I../../../include/uapi -I../../../lib
+LIBDIR := ../../../lib
+BPFOBJ := $(LIBDIR)/bpf/bpf.o
+
+CFLAGS += -Wall -O2 -lcap -I../../../include/uapi -I$(LIBDIR)
 
 test_objs = test_verifier test_tag test_maps test_lru_map test_lpm_map
 
 TEST_PROGS := $(test_objs) test_kmod.sh
 TEST_FILES := $(test_objs)
 
+.PHONY: all clean force
+
 all: $(test_objs)
 
-$(test_objs): ../../../lib/bpf/bpf.o
+# force a rebuild of BPFOBJ when its dependencies are updated
+force:
+
+$(BPFOBJ): force
+	$(MAKE) -C $(dir $(BPFOBJ))
+
+$(test_objs): $(BPFOBJ)
 
 include ../lib.mk