Include git2/odb_backend.h in git2.h

Since this isn't under sys/ we need to have it included in the main
git2.h file.

This fixes #2551.

include/git2.h

@@ -32,6 +32,7 @@
 #include "git2/notes.h"
 #include "git2/object.h"
 #include "git2/odb.h"
+#include "git2/odb_backend.h"
 #include "git2/oid.h"
 #include "git2/pack.h"
 #include "git2/patch.h"