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

See the commit 9173e8b604636633a8e3aca54bb56a437bffa718 (Create explain_format.c
and move relevant code there.) in PostgreSQL 18+.

Author: Marina Polyakova

signal_handler.c

@@ -11,6 +11,9 @@
 #include "pg_query_state.h"
 
 #include "commands/explain.h"
+#if PG_VERSION_NUM >= 180000
+#include "commands/explain_format.h"
+#endif
 #include "miscadmin.h"
 #if PG_VERSION_NUM >= 100000
 #include "pgstat.h"