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
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9740cdbe5e35c57abba1d3cfe082d8dd35118ca5

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