Use add_angle_brackets in ada_lookup_encoded_symbol

Joel recently pointed out add_angle_brackets to me.  This patch
changes one spot in ada-lang.c to use this function rather than doing
it on its own.

gdb/ChangeLog
2020-12-09  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_lookup_encoded_symbol): Use add_angle_brackets.