Use "const" more in ax-gdb.c

This changes a few spots in ax-gdb.c to use a "const char *" rather
than a non-const one.

gdb/ChangeLog
2020-11-10  Tom Tromey  <tom@tromey.com>

	* ax-gdb.c (gen_struct_elt_for_reference, gen_namespace_elt)
	(gen_maybe_namespace_elt, gen_aggregate_elt_ref, gen_expr): Use
	const.