kbuild: detect missing "WITH Linux-syscall-note" for uapi headers

UAPI headers licensed under GPL are supposed to have exception
"WITH Linux-syscall-note" so that they can be included into non-GPL
user space application code.

Unfortunately, people often miss to add it. Break 'make headers'
when any of exported headers lacks the exception note so that the
0-day bot can easily catch it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>