rcg: re-add need_to_load_IOP_first export symbol

This was removed in 8c01de98 but is needed by the RCG-generated
models.

src/drv/ExtraSymbols.symvers

+0x00000000      need_to_load_IOP_first  /       EXPORT_SYMBOL
 0x00000000	sci_register_session_cb	/	EXPORT_SYMBOL
 0x00000000	sci_kernel_virtual_address_of_mapping	/	EXPORT_SYMBOL
 0x00000000	sci_map_segment	/	EXPORT_SYMBOL