arch: Make the decoder decode() method virtual.

Change-Id: I60f0c4ffbd63069caaee190a78f007df79b61808
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52078
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/riscv/decoder.hh

@@ -75,7 +75,7 @@ class Decoder : public InstDecoder
     //when there is control flow.
     void moreBytes(const PCStateBase &pc, Addr fetchPC) override;
 
-    StaticInstPtr decode(PCStateBase &nextPC);
+    StaticInstPtr decode(PCStateBase &nextPC) override;
 };
 
 } // namespace RiscvISA