commit-graph: add missing forward declaration

Signed-off-by: NJonathan Tan <jonathantanmy@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

commit-graph.h

@@ -5,6 +5,8 @@
 #include "repository.h"
 #include "string-list.h"
 
+struct commit;
+
 char *get_commit_graph_filename(const char *obj_dir);
 
 /*