Kbuild: Add cpp_its_S in ksym_dep_filter

Kbuild: Add cpp_its_S in ksym_dep_filter

Add a new command cpp_its_S introduced in commit cf2a5e0bb4c6 ("MIPS:
Support generating Flattened Image Trees (.itb)") to ksym_dep_filter
handler - otherwise a warning is produced during the build of MIPS
platforms (when vmlinux.*.itb target is chosen).

Signed-off-by: Marcin Nowakowski <[email protected]>
Cc: Michal Marek <[email protected]>
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/15278/
Signed-off-by: James Hogan <[email protected]>