crypto: arm/poly1305 - Add prototype for poly1305_blocks_neon

This patch adds a prototype for poly1305_blocks_neon to slience
a compiler warning:

  CC [M]  arch/arm/crypto/poly1305-glue.o
../arch/arm/crypto/poly1305-glue.c:25:13: warning: no previous prototype for `poly1305_blocks_neon' [-Wmissing-prototypes]
 void __weak poly1305_blocks_neon(void *state, const u8 *src, u32 len, u32 hibit)
             ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>