ssl_tls: fix proper guards for accelerated ECDH

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index a1356e3..1572bcd 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -1510,7 +1510,7 @@
 
     return 0;
 }
-#endif /* MBEDTLS_ECDH_C */
+#endif /* MBEDTLS_PK_CAN_ECDH */
 
 /* RFC 8446 section 4.2
  *