ssl_tls13: fix guards for accel ECDH

These changes fix all failures found in test_suite_ssl

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 669a90a..a1356e3 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -1428,7 +1428,7 @@
     return ret;
 }
 
-#if defined(MBEDTLS_ECDH_C)
+#if defined(MBEDTLS_PK_CAN_ECDH)
 
 int mbedtls_ssl_tls13_read_public_ecdhe_share(mbedtls_ssl_context *ssl,
                                               const unsigned char *buf,