tls: replace ECDH_C guards with new helpers

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 4b73b41..2f46778 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -42,7 +42,7 @@
 
 #include "mbedtls/md.h"
 
-#if defined(MBEDTLS_ECDH_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_SOME_ECDH_OR_ECDHE_ANY_ENABLED)
 #include "mbedtls/ecdh.h"
 #endif