UPSTREAM: crypto: lib/blake2s - Move selftest prototype into header file

This patch fixes a missing prototype warning on blake2s_selftest.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit ce0d5d63e897cc7c3a8fd043c7942fc6a78ec6f4)
Bug: 152722841
Change-Id: Ieb19048d60f6c9dc36cefda8b09b3d5c95e9118e
Signed-off-by: Eric Biggers <ebiggers@google.com>