camera.device@3.2: Add missing field name to NotifyMsg

NotifyMsg's message union isn't actually instantiated.

Test: Compiles
Bug: 30985004
Change-Id: I214045208b624a1777ec0bd4353f268e4033166a
diff --git a/camera/device/3.2/types.hal b/camera/device/3.2/types.hal
index 3ce5037..505b6ba 100644
--- a/camera/device/3.2/types.hal
+++ b/camera/device/3.2/types.hal
@@ -635,7 +635,7 @@
          * Shutter message contents. Valid if type is MsgType::SHUTTER
          */
         ShutterMsg shutter;
-    };
+    } msg;
 
 };