PGDLLIMPORT-ify MyBgworkerEntry.

This was done in HEAD in commit 7d7eee8b,
but 9.3 needs it too for contrib/worker_spi.  Per buildfarm member narwhal.

src/include/postmaster/bgworker.h

@@ -82,7 +82,7 @@ typedef struct BackgroundWorker
 extern void RegisterBackgroundWorker(BackgroundWorker *worker);
 
 /* This is valid in a running worker */
-extern BackgroundWorker *MyBgworkerEntry;
+extern PGDLLIMPORT BackgroundWorker *MyBgworkerEntry;
 
 /*
  * Connect to the specified database, as the specified user.  Only a worker