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.

Conflicts:
	ChangeLog
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 78c748c..712053e 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -377,7 +377,7 @@
 int mbedtls_ssl_check_curve( const mbedtls_ssl_context *ssl, mbedtls_ecp_group_id grp_id );
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 int mbedtls_ssl_check_sig_hash( const mbedtls_ssl_context *ssl,
                                 mbedtls_md_type_t md );
 #endif