tools/bpf/runqslower: Rebuild libbpf.a on libbpf source change

Add missing dependency of $(BPFOBJ) to $(LIBBPF_SRC), so that running make
in runqslower/ will rebuild libbpf.a when there is change in libbpf/.

Fixes: 9c01546d ("tools/bpf: Add runqslower tool to tools/bpf")
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200204215037.2258698-1-songliubraving@fb.com

tools/bpf/runqslower/Makefile

@@ -75,7 +75,7 @@ $(OUTPUT)/vmlinux.h: $(VMLINUX_BTF_PATH) | $(OUTPUT) $(BPFTOOL)
 	fi
 	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF_PATH) format c > $@
 
-$(BPFOBJ): | $(OUTPUT)
+$(BPFOBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(OUTPUT)
 	$(Q)$(MAKE) $(submake_extras) -C $(LIBBPF_SRC)			       \
 		    OUTPUT=$(abspath $(dir $@))/ $(abspath $@)