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

Fixes clang warnings.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5228>

src/amd/compiler/aco_ir.h

@@ -748,7 +748,7 @@ private:
    };
 };
 
-class Block;
+struct Block;
 
 struct Instruction {
    aco_opcode opcode;