C: Fixup READONLY magic hashes for <5.14 #273

<5.14 c magic was missing causing test #16 to fail.
%main:: 'c' magic for tie:
  hv0 = gv_stashpvn("main", 4, 0);/* get main:: stash */
  /* AMT overload table for the stash hv0 is generated dynamically */
  SvSTASH_set((HV*)&sv_list[2], (HV*)hv0);
  SvREFCNT((SV*)hv0) += 1;
  ++PL_sv_objcount;