PGPRO-13320: include commands/explain_state.h for PostgreSQL 18+

See the commit c65bc2e1d14a2d4daed7c1921ac518f2c5ac3d17 (Make it possible for
loadable modules to add EXPLAIN options.) in PostgreSQL 18+.

Author: Marina Polyakova

pg_query_state.h

@@ -12,7 +12,11 @@
 
 #include <postgres.h>
 
+#if PG_VERSION_NUM >= 180000
+#include "commands/explain_state.h"
+#else
 #include "commands/explain.h"
+#endif
 #include "nodes/pg_list.h"
 #include "storage/procarray.h"
 #include "storage/shm_mq.h"