ssl_tls: fix guard symbols for EC accelerated tests

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 3de7a8f..5d2cf34 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -197,6 +197,11 @@
 #define MBEDTLS_PK_CAN_ECDSA_SOME
 #endif
 
+#if (defined(MBEDTLS_USE_PSA_CRYPTO) && defined(PSA_WANT_ALG_ECDH)) || \
+    (!defined(MBEDTLS_USE_PSA_CRYPTO) && defined(MBEDTLS_ECDH_C))
+#define MBEDTLS_PK_CAN_ECDH
+#endif
+
 /**
  * \brief           Types for interfacing with the debug module
  */