locking/refcounts: Do not force refcount_t usage as GPL-only export

The refcount_t protection on x86 was not intended to use the stricter
GPL export. This adjusts the linkage again to avoid a regression in
the availability of the refcount API.

Reported-by: Dave Airlie <airlied@gmail.com>
Fixes: 7a46ec0e

 ("locking/refcounts, x86/asm: Implement fast refcount overflow protection")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>