Move some things from builtins.h to new header files

This avoids that builtins.h has to include additional header files.

contrib/pageinspect/btreefuncs.c

@@ -34,6 +34,7 @@
 #include "miscadmin.h"
 #include "utils/builtins.h"
 #include "utils/rel.h"
+#include "utils/varlena.h"
 
 
 PG_FUNCTION_INFO_V1(bt_metap);

contrib/pageinspect/rawpage.c

@@ -25,6 +25,7 @@
 #include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/rel.h"
+#include "utils/varlena.h"
 
 PG_MODULE_MAGIC;