bluetooth: mesh: Fix proxy client compilation

When building proxy client without proxy server, it fails to find
the `identity` field in the `struct bt_mesh_subnet_keys` because the
latter only compiles when Proxy server is enabled. It should be
possible to compile Proxy Client without Proxy server.

This commit compiles the field if any of GATT features is enabled.

Fixes #75914

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>

subsys/bluetooth/mesh/subnet.h

@@ -74,7 +74,7 @@ struct bt_mesh_subnet {
 		struct bt_mesh_key net;         /* NetKey */
 		struct bt_mesh_net_cred msg;
 		uint8_t net_id[8];              /* Network ID */
-	#if defined(CONFIG_BT_MESH_GATT_PROXY)
+	#if defined(CONFIG_BT_MESH_GATT)
 		struct bt_mesh_key identity;    /* IdentityKey */
 	#endif
 		struct bt_mesh_key beacon;      /* BeaconKey */