commit-reach: correct accidental #include of C file

Without this change, the build breaks with clang:

 libgit/ref-filter.pic.o: multiple definition of 'filter_refs'
 libgit/commit-reach.pic.o: previous definition here

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-reach.c

diff --git a/commit-reach.c b/commit-reach.c
index c996524..86715c1 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -4,7 +4,7 @@
 #include "decorate.h"
 #include "prio-queue.h"
 #include "tree.h"
-#include "ref-filter.c"
+#include "ref-filter.h"
 #include "revision.h"
 #include "tag.h"
 #include "commit-reach.h"