gdb/solib: make implementation of solib_ops::open_symbol_file_object optional The only solib implementation that implements open_symbol_file_object is SVR4. All others just return 0. Make it optional, to avoid having these empty functions. Change-Id: I835197a73323d39231d071f9a9eaac2553f10726 Reviewed-By: Guinevere Larsen <guinevere@redhat.com>