Add codeowners for Codegen/Common/GPU (iree-org#15498)

I noticed that because it is nested under Codegen/Common, currently the
only reviewers that get assigned for changes to Codegen/Common/GPU are
hanhanW and dcaballe. Here I am just adding antiagainst and myself.
(removed mentions because otherwise it sends frequent pings)

.github/CODEOWNERS

@@ -62,6 +62,7 @@
 /compiler/src/iree/compiler/ @benvanik
 /compiler/src/iree/compiler/Codegen/ @MaheshRavishankar
 /compiler/src/iree/compiler/Codegen/Common @hanhanW @dcaballe
+/compiler/src/iree/compiler/Codegen/Common/GPU @antiagainst @qedawkins
 /compiler/src/iree/compiler/Codegen/LLVMCPU/ @dcaballe @hanhanW @MaheshRavishankar
 /compiler/src/iree/compiler/Codegen/LLVMGPU/ @MaheshRavishankar
 /compiler/src/iree/compiler/Codegen/SPIRV/ @antiagainst @MaheshRavishankar