codegen: Change pointer casting to remove type-punning warnings
The existing code was generating code with undefined results that modern compilers warn about:
accounts-generated.c:204:23: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing] (GDBusArgInfo **) &_accounts_accounts_method_info_list_cached_users_OUT_ARG_pointers,
(Trivial backport of !309 (merged) from master.)