gpu-compute: mark functions with override if replacing virtual

The clang compiler is more stringent than the recent versions of
GCC when dealing with overrides. This changeset adds the specifier
to the methods which need it to silence the compiler.

src/arch/hsail/insts/gpu_static_inst.hh

@@ -54,7 +54,7 @@ namespace HsailISA
     {
       public:
         HsailGPUStaticInst(const BrigObject *obj, const std::string &opcode);
-        void generateDisassembly();
+        void generateDisassembly() override;
         int instSize() const override { return sizeof(RawMachInst); }
         bool isValid() const override { return true; }