Add extern declarations for Bison global variables

Add extern declarations for Bison global variables

This adds extern declarations for some global variables produced by
Bison that are not already declared in its generated header file.
This is a workaround to be able to add -Wmissing-variable-declarations
to the global set of warning options in the near future.

Another longer-term solution would be to convert these grammars to
"pure" parsers in Bison, to avoid global variables altogether.  Note
that the core grammar is already pure, so this patch did not need to
touch it.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org