Remove one use of sprintf in ada-lang.c (print_it_exception)

Remove one use of sprintf in ada-lang.c (print_it_exception)

This is just a minor cleanup of an ARI violation.  As this affects
code that is normally unreachable (you'd need a runtime where
the unit providing the exception hook has been compiled without
debugging information), I tested this manually by commenting out
the code just above, thus forcing this part of the code to be
exercised.

gdb/ChangeLog:

        * ada-lang.c (print_it_exception): Avoid use of sprintf.

Tested on x86_64-linux.