Create API for mbedtls_ssl_conf_sig_hashes().

Not implemented yet.
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index e4f0686..40c2d4c 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1836,9 +1836,7 @@
           MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED */
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1_2)
-#if defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED) ||                       \
-    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED) ||                     \
-    defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
 static int ssl_parse_signature_algorithm( mbedtls_ssl_context *ssl,
                                           unsigned char **p,
                                           unsigned char *end,
@@ -1884,12 +1882,9 @@
 
     return( 0 );
 }
-#endif /* MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED ||
-          MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED ||
-          MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED */
 #endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
 
-
 #if defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED)
 static int ssl_get_ecdh_params_from_cert( mbedtls_ssl_context *ssl )