ucontext-cp: Mark a non-exported function as static

Functions that are not used outside the translation unit should be
static.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Link: https://lore.kernel.org/r/20221124162633.3856761-7-ammar.faizi@intel.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>