Mark MdSMgrId PGDLLIMPORT

Needed so loadable modules can reference the builtin md smgr id across
the DLL boundary on Windows; no-op elsewhere.

Author: dutow

src/include/storage/md.h

@@ -24,7 +24,7 @@ extern PGDLLIMPORT PgAioHandleCallbacks aio_md_readv_cb;
 
 /* registration function for md storage manager */
 extern void mdsmgr_register(void);
-extern SMgrId MdSMgrId;
+extern PGDLLIMPORT SMgrId MdSMgrId;
 
 /* md storage manager functionality */
 extern void mdinit(void);