Correct variable names in embed.fnc for hv_free_ent and hv_free_ent_ret.

Make the second variable name in embed.fnc match those used in the
actual function declaration.  This will matter if we add in 'entry'
to PERL_ARGS_ASSERT_HV_FREE_ENT_RET.  Also regen headers (only proto.h
is affected) to match.