Add PGDLLEXPORT on exposed functions.

The PG_FUNCTION_INFO_V1() marco also takes care of the extra annotation for the
generated info function but not the exposed function itself.

Similarly, the _PG_init() and _PG_fini() functions needs to be exported
(although the _PG_fini() is currently not called by postgres).

This should fix the runtime errors on Windows reported by github user suprimex.

CONTRIBUTORS.md

@@ -13,3 +13,4 @@
   * github user seqizz
   * github user akovac
   * Romain DEP
+  * github user suprimex

pg_qualstats.c

@@ -106,18 +106,18 @@ typedef enum pgqsVersion
 
 /*---- Function declarations ----*/
 
-void		_PG_init(void);
-void		_PG_fini(void);
-
-extern Datum pg_qualstats_reset(PG_FUNCTION_ARGS);
-extern Datum pg_qualstats(PG_FUNCTION_ARGS);
-extern Datum pg_qualstats_2_0(PG_FUNCTION_ARGS);
-extern Datum pg_qualstats_names(PG_FUNCTION_ARGS);
-extern Datum pg_qualstats_names_2_0(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT void		_PG_init(void);
+extern PGDLLEXPORT void		_PG_fini(void);
+
+extern PGDLLEXPORT Datum pg_qualstats_reset(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats_2_0(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats_names(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats_names_2_0(PG_FUNCTION_ARGS);
 static Datum pg_qualstats_common(PG_FUNCTION_ARGS, pgqsVersion api_version,
 								 bool include_names);
-extern Datum pg_qualstats_example_query(PG_FUNCTION_ARGS);
-extern Datum pg_qualstats_example_queries(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats_example_query(PG_FUNCTION_ARGS);
+extern PGDLLEXPORT Datum pg_qualstats_example_queries(PG_FUNCTION_ARGS);
 
 PG_FUNCTION_INFO_V1(pg_qualstats_reset);
 PG_FUNCTION_INFO_V1(pg_qualstats);