Use obstack_strdup in ada-lang.c

I happened across a spot that was still using obstack_alloc and
strcpy, rather than obstack_strdup.  This patch makes the obvious fix.

gdb/ChangeLog
2020-02-19  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (cache_symbol): Use obstack_strdup.