Makefile: add thread-utils.h to LIB_H

Starting with commit v1.7.8-165-g0579f91, grep.h includes
thread-utils.h, so the latter has to be added to LIB_H.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -576,6 +576,7 @@ LIB_H += streaming.h
 LIB_H += string-list.h
 LIB_H += submodule.h
 LIB_H += tag.h
+LIB_H += thread-utils.h
 LIB_H += transport.h
 LIB_H += tree.h
 LIB_H += tree-walk.h