pgsql: MyBackendId now needs to be PGDLLIMPORT, so that contrib modules

Log Message:
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MyBackendId now needs to be PGDLLIMPORT, so that contrib modules can
access it on Windows. Per buildfarm.

Modified Files:
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pgsql/src/include/storage:
backendid.h (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/backendid.h?r1=1.23&r2=1.24)