contrib/amcheck needs RecentGlobalXmin to be PGDLLIMPORT'ified.

Per buildfarm.  Maybe some of the other xmin variables in snapmgr.h
ought to get this too, but for the moment I'm just interested in
un-breaking the buildfarm.

src/include/utils/snapmgr.h

@@ -58,7 +58,7 @@ extern bool FirstSnapshotSet;
 
 extern TransactionId TransactionXmin;
 extern TransactionId RecentXmin;
-extern TransactionId RecentGlobalXmin;
+extern PGDLLIMPORT TransactionId RecentGlobalXmin;
 extern TransactionId RecentGlobalDataXmin;
 
 extern Snapshot GetTransactionSnapshot(void);