C: cast xmg_stash symbol to (HV*)

5.18.2/x86_64-linux/CORE/sv.h:1297:51: warning: assignment from incompatible pointer type [enabled by default]
  (((XPVMG*)  SvANY(sv))->xmg_stash = (val)); } STMT_END
                                                   ^
ccode93ib_3.c:27393:2: note: in expansion of macro ‘SvSTASH_set’
  SvSTASH_set(hv0, &sv_list[8]);
  ^
test: less todo with 27,29,i71