Fix unused src tracking for generated .o files

*.o files that are passed in via LOCAL_GENERATED_SOURCES are added
directly to all_objects, they are not mixed with the normal_objects that
we track. So omit them from they my_gen_src_files list so that we don't
warn that they're unused.

Change-Id: I94b85504032e70fbcc00207d6200557700dd0a89