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
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ef8e6b2c2ba05f27b7bbfcb1a180db07d1c274ee

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