Use "bool" in ada-lang.c

Christian suggested switching an "int" in ada-lang.c to "bool"
instead.  This patch makes this change.  Tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-03-04  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (struct match_data) <found_sym>: Now bool.
	(aux_add_nonlocal_symbols): Update.
	(ada_add_block_symbols): Change "found_sym" to bool.