pgsql: Add GUC variables for stat tracking and timeout as PGDLLIMPORT

Add GUC variables for stat tracking and timeout as PGDLLIMPORT

This helps integration of extensions with Windows. The following
parameters are changed:
- idle_in_transaction_session_timeout (9.6 and newer versions)
- lock_timeout
- statement_timeout
- track_activities
- track_counts
- track_functions

Author: Pascal Legrand
Reviewed-by: Amit Kamila, Julien Rouhaud, Michael Paquier
Discussion: /messages/by-id/1579298868581-0.post@n3.nabble.com
Backpatch-through: 9.4

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/62c9b522311afd791fd92fe46a294f7e21f10540

Modified Files
--------------
src/include/pgstat.h | 6 +++---
src/include/storage/proc.h | 6 +++---
2 files changed, 6 insertions(+), 6 deletions(-)