Makefile: add merge_recursive.h to LIB_H

When modifying merge-recursive.h, for example builtin-merge-recursive.c
have to be recompiled which was not true till now, causing various
runtime errors using an incremental build.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -358,6 +358,7 @@ LIB_H += list-objects.h
 LIB_H += ll-merge.h
 LIB_H += log-tree.h
 LIB_H += mailmap.h
+LIB_H += merge-recursive.h
 LIB_H += object.h
 LIB_H += pack.h
 LIB_H += pack-refs.h