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 <marcin.nowakowski@imgtec.com>
Cc: Michal Marek <mmarek@suse.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15278/
Signed-off-by: James Hogan <james.hogan@imgtec.com>

scripts/Kbuild.include

@@ -284,7 +284,7 @@ ksym_dep_filter =                                                            \
 	    $(CPP) $(call flags_nodeps,c_flags) -D__KSYM_DEPS__ $< ;;        \
 	  as_*_S|cpp_s_S)                                                    \
 	    $(CPP) $(call flags_nodeps,a_flags) -D__KSYM_DEPS__ $< ;;        \
-	  boot*|build*|*cpp_lds_S|dtc|host*|vdso*) : ;;                      \
+	  boot*|build*|cpp_its_S|*cpp_lds_S|dtc|host*|vdso*) : ;;            \
 	  *) echo "Don't know how to preprocess $(1)" >&2; false ;;          \
 	esac | tr ";" "\n" | sed -rn 's/^.*=== __KSYM_(.*) ===.*$$/KSYM_\1/p'