Remove PGDLLIMPORT marker from __pg_log_level

Per discussion with Tom Lane and Andres Freund.  I have misunderstood
the intention behind the choice done in 9a374b7.

Discussion: https://postgr.es/m/20220512153737.6kbbcf4qyvwgq4s2@alap3.anarazel.de

Author: michaelpq

src/include/common/logging.h

@@ -51,7 +51,7 @@ enum pg_log_level
 /*
  * __pg_log_level is the minimum log level that will actually be shown.
  */
-extern PGDLLIMPORT enum pg_log_level __pg_log_level;
+extern enum pg_log_level __pg_log_level;
 
 /*
  * A log message can have several parts.  The primary message is required,