Fix #ifdef inconsistency

fixes #310

Actually all key exchanges that use a certificate use signatures too, and
there is no key exchange that uses signatures but no cert, so merge those two
flags.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 13e5b0c..f53c800 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -529,7 +529,7 @@
     mbedtls_x509_crl *ca_crl;       /*!< trusted CAs CRLs                   */
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
     const int *sig_hashes;          /*!< allowed signature hashes           */
 #endif
 
@@ -1548,7 +1548,7 @@
                               const mbedtls_ecp_group_id *curves );
 #endif /* MBEDTLS_ECP_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 /**
  * \brief          Set the allowed hashes for signatures during the handshake.
  *                 (Default: all available hashes.)
@@ -1569,7 +1569,7 @@
  */
 void mbedtls_ssl_conf_sig_hashes( mbedtls_ssl_config *conf,
                                   const int *hashes );
-#endif /* MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /**