Mark ReplicationSlotCtl as PGDLLIMPORT.

Also MyReplicationSlot, in branches where it wasn't already.

This was discussed in the thread that resulted in c572599c, but
for some reason nobody pulled the trigger.  Now that we have another
request for the same thing, we should just do it.

Craig Ringer

Discussion: https://postgr.es/m/CAMsr+YFTsq-86MnsNng=mPvjjh5EAbzfMK0ptJPvzyvpFARuRg@mail.gmail.com
Discussion: https://postgr.es/m/345138875.20190611151943@cybertec.at