kati doesn't support .SECONDEXPANSION

The `.SECONDEXPANSION` special target does not have any effect in kati other than to print a warning that it is not supported.

This feature is used heavily by Linux's build system. The rule `$(obj)/%.o: $(src)/%.c ...` in scripts/Makefile.build also takes `$$(objtool_dep)` as an input, which without secondary expansion will be treated as the literal file `$(objtool_dep)`, which does not exist and cannot be made, so C files are instead compiled with the builtin rule, which doesn't work.