pgsql: 'PGDLLIMPORT' ShmemVariableCache, needed for pg_migrator.so

Log Message:
-----------
'PGDLLIMPORT' ShmemVariableCache, needed for pg_migrator.so function
linkage on Win32.

Tested by Hiroshi Saito

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