UPSTREAM: crypto: lib/chacha20poly1305 - Add missing function declaration

This patch adds a declaration for chacha20poly1305_selftest to
silence a sparse warning.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
(cherry picked from commit 06cc2afbbdf9a9e8df3e2f8db724997dd6e1b4ac)
Bug: 152722841
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I9dc1c6df2e73e6a60d81f24384e8363309d6d109