Makefile: add patch-ids.h back in.

I lost it by mistake while shuffling the gitattributes series which
originally was on top of the subproject topic onto the master branch.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -283,7 +283,7 @@ LIB_H = \
 	diff.h object.h pack.h pkt-line.h quote.h refs.h list-objects.h sideband.h \
 	run-command.h strbuf.h tag.h tree.h git-compat-util.h revision.h \
 	tree-walk.h log-tree.h dir.h path-list.h unpack-trees.h builtin.h \
-	utf8.h reflog-walk.h attr.h
+	utf8.h reflog-walk.h patch-ids.h attr.h
 
 DIFF_OBJS = \
 	diff.o diff-lib.o diffcore-break.o diffcore-order.o \