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 <[e-mail]>
Signed-off-by: William Tu <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/lib/bpf/Makefile

···
 		$(call do_install,bpf.h,$(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)