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
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1ef7332b78f55f0503a8c882886d2979c2f94c62

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