Bluetooth: GATT: Fix taking address of packed member of struct sc_data.

Fix GCC9 warning "warning: taking address of packed member of
'struct sc_data' may result in an unaligned pointer value"
Issue is that the on-air structure of sc_data was re-used for the gatt
service changed data.

Added build assert because data is stored in settings, so the structure
should be the same size to be compatible.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>