Mark a few parallelism-related variables with PGDLLIMPORT.

Back-patch commit 09a65f5a into the 9.6 and 10 branches.
Needed to support back-patch of commit 2cd4e835 on Windows.

Discussion: http://postgr.es/m/20190604011354.GD1529@paquier.xyz

src/include/access/parallel.h

@@ -48,8 +48,8 @@ typedef struct ParallelContext
 } ParallelContext;
 
 extern volatile bool ParallelMessagePending;
-extern int	ParallelWorkerNumber;
-extern bool InitializingParallelWorker;
+extern PGDLLIMPORT int ParallelWorkerNumber;
+extern PGDLLIMPORT bool InitializingParallelWorker;
 
 #define		IsParallelWorker()		(ParallelWorkerNumber >= 0)