Apply PGDLLIMPORT markings to some GUC variables

Hi hackers,

I noticed that in commit d3cc5ffe81f6 some GUC variables were moved to
header files. According to the commit message in 8ec569479, all
variables in header files must be marked with PGDLLIMPORT, am I right?

I've made a patch that adds missing PGDLLIMPORTs, please, take a look.

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Best regards,
Sofia Kopikova
Postgres Professional: http://www.postgrespro.com