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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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2ff2baa220fc3eb5fba8de2864b619e648e86f17

Modified Files
--------------
src/include/miscadmin.h       |  2 +-
src/include/optimizer/cost.h  | 30 +++++++++++++++---------------
src/include/optimizer/paths.h |  8 ++++----
src/include/utils/guc.h       |  2 +-
4 files changed, 21 insertions(+), 21 deletions(-)