samples/bpf: Partially fixes the bpf.o build

Do not build lib/bpf/bpf.o with this Makefile but use the one from the
library directory.  This avoid making a buggy bpf.o file (e.g. missing
symbols).

This patch is useful if some code (e.g. Landlock tests) needs both the
bpf.o (from tools/lib/bpf) and the bpf_load.o (from samples/bpf).
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/Makefile

@@ -201,13 +201,16 @@ CLANG_ARCH_ARGS = -target $(ARCH)
 endif
 
 # Trick to allow make to be run from this directory
-all:
+all: $(LIBBPF)
 	$(MAKE) -C ../../ $(CURDIR)/
 
 clean:
 	$(MAKE) -C ../../ M=$(CURDIR) clean
 	@rm -f *~
 
+$(LIBBPF): FORCE
+	$(MAKE) -C $(dir $@) $(notdir $@)
+
 $(obj)/syscall_nrs.s:	$(src)/syscall_nrs.c
 	$(call if_changed_dep,cc_s_c)