Apply PGDLLIMPORT markings to some GUC variables

Apply PGDLLIMPORT markings to some GUC variables

According to the commit message in 8ec569479, we must have all variables
in header files marked with PGDLLIMPORT. In commit d3cc5ffe81f6 some
variables were moved from launch_backend.c file to several header files.

This adds PGDLLIMPORT to moved variables.

Author: Sofia Kopikova <s.kopikova@postgrespro.ru>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/e0b17014-5319-4dd6-91cd-93d9c8fc9539%40postgrespro.ru