devcontainer.json: set permissions for /dev/kvm on every start

This prevents gem5 from erroring out after closing and re-opening VS Code.

.devcontainer.json

@@ -13,5 +13,6 @@
 			]
 		}
 	},
-	"remoteUser": "simbricks"
+	"remoteUser": "simbricks",
+	"postStartCommand": "sudo chmod o+rw /dev/kvm"
 }