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: https://postgr.es/m/1579298868581-0.post@n3.nabble.com
Backpatch-through: 9.4

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dbe405b7859c68a4927afd8334059cf9348afbeb

Modified Files
--------------
src/include/pgstat.h       | 6 +++---
src/include/storage/proc.h | 4 ++--
2 files changed, 5 insertions(+), 5 deletions(-)