pgsql: Backport: Mark assorted GUC variables as PGDLLIMPORT.

Backport: Mark assorted GUC variables as PGDLLIMPORT.

This backpatches 935dee9ad5a8d12f4d3b772a6e6c99d245e5ad44 to the
the branches requested by extension authors.

Original-Author: Metin Doslu
Original-Committer: Robert Haas
Author: Brian Cloutier

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/83fce670ea395c4e65c34886518273bf478615c4

Modified Files
--------------
src/include/optimizer/cost.h | 28 ++++++++++++++--------------
src/include/optimizer/paths.h | 6 +++---
src/include/utils/guc.h | 2 +-
3 files changed, 18 insertions(+), 18 deletions(-)