samples: Bluetooth: hci_ipc: Use LTO when building ISO features

Use Link Time Optimisation (LTO) when building Controller
with Isochronous channels support.

nRF53x SoCs has 256KB flash and using speed optimisation,
which is the default today, overflows flash when adding
new features related to ISO support.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>