Added missing clean-up to the clean target of the Makefile
Added missing clean-up to the clean target of the Makefile

SymbiFlow/arachne-pnr,cseed/arachne-pnr,SymbiFlow/arachne-pnr,cseed/arachne-pnr,cseed/arachne-pnr,SymbiFlow/arachne-pnr