New header varatt.h split off from postgres.h

This new header contains all the variable-length data types support
(TOAST support) from postgres.h, which isn't needed by large parts of
the backend code.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/ddcce239-0f29-6e62-4b47-1f8ca742addf%40enterprisedb.com

src/backend/utils/adt/network_gist.c

@@ -51,6 +51,7 @@
 #include "access/stratnum.h"
 #include "utils/builtins.h"
 #include "utils/inet.h"
+#include "varatt.h"
 
 /*
  * Operator strategy numbers used in the GiST inet_ops opclass