Revert "Makefile: Do not quote value for CONFIG_CC_IMPLICIT_FALLTHROUGH"

This reverts commit cd8c917a.

Commit 129ab0d2 ("kbuild: do not quote string values in
include/config/auto.conf") provided the final solution.

Now reverting the temporary workaround.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>