Simplify use of map_matching_symbols in ada-lang.c

Simplify use of map_matching_symbols in ada-lang.c

I noticed that ada-lang.c creates a lambda to call
aux_add_nonlocal_symbols.  However, this code can be simplified a bit
by changing match_data to implement operator(), and then simply
passing the object as the callback.  That is what this patch
implements.

gdb/ChangeLog
2021-03-26  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (struct match_data): Add operator().
	(match_data::operator()): Rename from aux_add_nonlocal_symbols.
	(callback): Remove 'callback'.