kasan: export kasan_poison as GPL

kasan: export kasan_poison as GPL

KASAN uses EXPORT_SYMBOL_GPL for symbols whose exporting is only required
for KASAN tests when they are built as a module.

kasan_poison is one on those symbols, so export it as GPL.

Link: https://lkml.kernel.org/r/171d0b8b2e807d04cca74f973830f9b169e06fb8.1703188911.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>