modules: hal_silabs: Set multiprotocol define based on Kconfig

Set `SL_RAIL_LIB_MULTIPROTOCOL_SUPPORT` to `1` when Rail
Multiprotocol Kconfig is enabled.

This is required to run the Bluetooth multiprotocol initialization
code during `sl_btctrl_init`.

Signed-off-by: James Smith <james@loopj.com>