Add PGDLLIMPORT to ScanKeywords and NumScanKeywords.

Per buildfarm, this is now needed by contrib/pg_stat_statements.

src/include/parser/keywords.h

@@ -28,8 +28,8 @@ typedef struct ScanKeyword
 	int16		category;		/* see codes above */
 } ScanKeyword;
 
-extern const ScanKeyword ScanKeywords[];
-extern const int NumScanKeywords;
+extern PGDLLIMPORT const ScanKeyword ScanKeywords[];
+extern PGDLLIMPORT const int NumScanKeywords;
 
 extern const ScanKeyword *ScanKeywordLookup(const char *text,
 				  const ScanKeyword *keywords,