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>

Makefile

@@ -1508,7 +1508,7 @@ endif
 # make distclean Remove editor backup files, patch leftover files and the like
 
 # Directories & files removed with 'make clean'
-CLEAN_FILES += include/ksym vmlinux.symvers modules-only.symvers \
+CLEAN_FILES += vmlinux.symvers modules-only.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
 	       compile_commands.json .thinlto-cache rust/test rust/doc \
 	       rust-project.json .vmlinux.objs .vmlinux.export.c