cmake: update FindDeprecated for SOURCES

PR#51049 deprecated SOURCES but placed it below the
Deprecated_FIND_COMPONENTS check causing following message to be printed
when building for the unit_testing board:
-- The following deprecated component(s) could not be found: SOURCES

Fix this by placing the deprecated SOURCES handling at proper location
and append SOURCES to Deprecated_FIND_COMPONENTS list.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>