os: Squash missing declaration warning for timingsafe_memcmp

timingsafe_memcmp.c:21:1: warning: no previous prototype for ‘timingsafe_memcmp’ [-Wmissing-prototypes]
 timingsafe_memcmp(const void *b1, const void *b2, size_t len)

Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 5c44169c)