[PGPRO-4224] Include guc.h instead of declaring guc variable

Windows build does some magic to export variables from postgres.exe and import
them into the extension. So it is better to include backend include file than
to redeclare variable in the extension code

Author: vbwagner

src/rumsort.c

@@ -52,9 +52,13 @@
  * Below are copied definitions from src/backend/utils/sort/tuplesort.c.
  */
 
-/* GUC variables */
+/* GUC variables shouldn't be declared explicitely.
+  Rather corresponigng include file should be include because it
+  contains neccessary Windows export/import magic. And part of this
+  magic should be done during postgres.exe compilation
+*/
 #ifdef TRACE_SORT
-extern bool	trace_sort;
+#include <utils/guc.h>
 #endif
 
 typedef struct