gpu-compute: Add missing overrides

These missing overrides were causing compilations errors with the Clang
11 compiler:
https://www.mail-archive.com/gem5-dev@gem5.org/msg39683.html

Change-Id: Ib5e7096ab9a7a8505bcc848ff3f08674f7f289ce
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47899
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/gpu-compute/gpu_command_processor.hh

@@ -135,7 +135,7 @@ class GPUCommandProcessor : public DmaVirtDevice
     typedef void (DmaDevice::*DmaFnPtr)(Addr, int, Event*, uint8_t*, Tick);
     void initABI(HSAQueueEntry *task);
     HSAPacketProcessor *hsaPP;
-    void translateOrDie(Addr vaddr, Addr &paddr);
+    void translateOrDie(Addr vaddr, Addr &paddr) override;
 
     /**
      * Perform a DMA read of the read_dispatch_id_field_base_byte_offset