Remember suitable hash function for any signature algorithm.

This commit changes `ssl_parse_signature_algorithms_ext` to remember
one suitable ( := supported by client and by our config ) hash
algorithm per signature algorithm.

It also modifies the ciphersuite checking function
`ssl_ciphersuite_match` to refuse a suite if there
is no suitable hash algorithm.

Finally, it adds the corresponding entry to the ChangeLog.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 495e02c..eb0cfdf 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -531,6 +531,7 @@
 /* Defined in ssl_internal.h */
 typedef struct mbedtls_ssl_transform mbedtls_ssl_transform;
 typedef struct mbedtls_ssl_handshake_params mbedtls_ssl_handshake_params;
+typedef struct mbedtls_ssl_sig_hash_set_t mbedtls_ssl_sig_hash_set_t;
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 typedef struct mbedtls_ssl_key_cert mbedtls_ssl_key_cert;
 #endif