aco: Don't declare 'Block' as class, but define as struct.

From Oschowa
536339b0dda33241d21a0e045681419ca46fc812 in mainline mesa

reduces clang warnings

lib/mesa/src/amd/compiler/aco_ir.h

@@ -747,7 +747,7 @@ private:
    };
 };
 
-class Block;
+struct Block;
 
 struct Instruction {
    aco_opcode opcode;