skip scripts/mod files when detecting changed objs

In some environments, some files in scripts/mod (devicetable-offsets.s
and file2alias.o) are always getting rebuilt, and thus get incorrectly
added to the changed_objs file, resulting in the following error:

 strip:/root/.kpatch/3.10.0-115.el7.x86_64/obj2/scripts/mod/devicetable-offsets.s: File format not recognized