Add missing dependency in cypher_expr.c (#1256)

The function contain_vars_of_level() was missing a header.
Added that header to prevent a warning during compilation.

Author: dehowef

src/backend/parser/cypher_expr.c

@@ -31,6 +31,7 @@
 #include "nodes/nodes.h"
 #include "nodes/parsenodes.h"
 #include "nodes/value.h"
+#include "optimizer/optimizer.h"
 #include "optimizer/tlist.h"
 #include "parser/parse_coerce.h"
 #include "parser/parse_collate.h"