libbpf: add libbpf_util.h to header install.

The libbpf_util.h is used by xsk.h, so add it to
the install headers.
Reported-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/Makefile

@@ -230,6 +230,7 @@ install_headers:
 		$(call do_install,bpf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,libbpf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,btf.h,$(prefix)/include/bpf,644); \
+		$(call do_install,libbpf_util.h,$(prefix)/include/bpf,644); \
 		$(call do_install,xsk.h,$(prefix)/include/bpf,644);
 
 install_pkgconfig: $(PC_FILE)