cmake/sgm776: Add missing executable name extension

The extension for the source file config_scmi_power_domain is missing.
Cmake throws a `Policy CMP0115` warning when building the firmware for
sgm776.

This patch adds the missing file extension in ramfw/CMakeList.txt.

Change-Id: Ib0c13b289c1f21d486c017714df06b9be7868e34
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>