selftests/bpf: Re-generate vmlinux.h and BPF skeletons if bpftool changed

Trigger vmlinux.h and BPF skeletons re-generation if detected that bpftool was
re-compiled. Otherwise full `make clean` is required to get updated skeletons,
if bpftool is modified.

Fixes: acbd0620 ("selftests/bpf: Add vmlinux.h selftest exercising tracing of syscalls")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210318194036.3521577-11-andrii@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -232,7 +232,7 @@ $(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)                \
 		    DESTDIR=$(HOST_SCRATCH_DIR)/ prefix= all install_headers
 endif
 
-$(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) | $(BPFTOOL) $(INCLUDE_DIR)
+$(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR)
 ifeq ($(VMLINUX_H),)
 	$(call msg,GEN,,$@)
 	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
@@ -357,7 +357,8 @@ $(TRUNNER_BPF_OBJS): $(TRUNNER_OUTPUT)/%.o:				\
 
 $(TRUNNER_BPF_SKELS): $(TRUNNER_OUTPUT)/%.skel.h:			\
 		      $(TRUNNER_OUTPUT)/%.o				\
-		      | $(BPFTOOL) $(TRUNNER_OUTPUT)
+		      $(BPFTOOL)					\
+		      | $(TRUNNER_OUTPUT)
 	$$(call msg,GEN-SKEL,$(TRUNNER_BINARY),$$@)
 	$(Q)$$(BPFTOOL) gen skeleton $$< > $$@
 endif