fp: Use a forward declaration in fused.h

It's permissible to forward declare here, so we can do so and eliminate
a direct header dependency

src/common/fp/fused.h

@@ -6,10 +6,10 @@
 
 #pragma once
 
-#include "common/fp/unpacked.h"
-
 namespace Dynarmic::FP {
 
+struct FPUnpacked;
+
 FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2);
 
 } // namespace Dynarmic::FP