gpu-compute: Add missing override in render driver

This fixes the build error in the clang-11 compiler check for GCN3_X86.

Change-Id: I2245589182b80811b8bc07409196adca98899213
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47479
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/gpu-compute/gpu_render_driver.hh

@@ -46,7 +46,7 @@ class GPURenderDriver final : public EmulatedDriver
     int open(ThreadContext *tc, int mode, int flags) override;
 
     int
-    ioctl(ThreadContext *tc, unsigned req, Addr buf)
+    ioctl(ThreadContext *tc, unsigned req, Addr buf) override
     {
         return -EBADF;
     }