drivers/nble: Fix included services

This fixes the regression of included services in nble.
Because user_data of BT_GATT_INCLUDE_SERVICE points to the first
attribute of service to include, and the bt_gatt_attr_read uses
memcpy, the pointer to pointer is needed to copy the address contained
in user_data to the buffer.
With this patch, included services work on nble as well as Zephyr.

Change-Id: If360014675c399fafc365185d5c1e41c913c8109
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>