kbuild: rename cmd_cc_i_c to cmd_cpp_i_c

This command just preprocesses .c files into .i files, so cmd_cpp_i_c
seems more suitable.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.com>