vhost-user-gpu: Drop trailing json comma

Trailing comma is not valid json:

$ cat contrib/vhost-user-gpu/50-qemu-gpu.json.in | jq
parse error: Expected another key-value pair at line 5, column 1

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 7f5dd2ac9f3504e2699f23e69bc3d8051b729832.1568925097.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

contrib/vhost-user-gpu/50-qemu-gpu.json.in

@@ -1,5 +1,5 @@
 {
   "description": "QEMU vhost-user-gpu",
   "type": "gpu",
-  "binary": "@libexecdir@/vhost-user-gpu",
+  "binary": "@libexecdir@/vhost-user-gpu"
 }