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>