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.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
pgsql/src/include/commands:
trigger.h (r1.66 -> r1.66.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/trigger.h?r1=1.66&r2=1.66.2.1)