pgsql: Mark SessionReplicationRole as PGDLLIMPORT so it can be used from

Log Message:
-----------
Mark SessionReplicationRole as PGDLLIMPORT so it
can be used from Slony functions.

Per report from Hiroshi Saito.

Modified Files:
--------------
pgsql/src/include/commands:
trigger.h (r1.67 -> r1.68)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h?r1=1.67&r2=1.68)