crypto: ecdh - fix typo in KPP dependency of CRYPTO_ECDH

This fixes a typo in the CRYPTO_KPP dependency of CRYPTO_ECDH.

Fixes: 3c4b2390 ("crypto: ecdh - Add ECDH software support")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>