kbuild: remove include/ksym from CLEAN_FILES

This is a remnant of commit 5e9e95cc ("kbuild: implement
CONFIG_TRIM_UNUSED_KSYMS without recursion").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>