Makefile: Add cache-tree.h to the headers list

The dependency was missing.
Signed-off-by: NDmitry V. Levin <ldv@altlinux.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -281,7 +281,7 @@ LIB_FILE=libgit.a
 XDIFF_LIB=xdiff/lib.a
 
 LIB_H = \
-	archive.h blob.h cache.h commit.h csum-file.h delta.h grep.h \
+	archive.h blob.h cache.h cache-tree.h commit.h csum-file.h delta.h grep.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 \