ira: Fix s390 build

My g:01f3e6a4 broke the s390
build because the rclass variable was still needed by the
IRA_HARD_REGNO_ADD_COST_MULTIPLIER code.

gcc/
	* ira-costs.c (ira_tune_allocno_costs): Fix missing rclass
	definition in IRA_HARD_REGNO_ADD_COST_MULTIPLIER code.