tests input/kbd_matrix: Add missing overlays for native_sim

During the hwmv1->v2 transition, overlays from a base
board were made to be shared with the variants.
So at that time all overlays for variants which were
just copies of the base overlay were removed.
After that in
zephyrproject-rtos#71149
this shared/merged overlay behaviour was reverted,
but not all tests were fixed.

This is one of those. Let's fix it.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

Author: aescolar

tests/drivers/input/kbd_matrix/boards/native_sim_native_64.overlay

@@ -0,0 +1,6 @@
+/*
+ * Copyright (c) 2024 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "native_sim.overlay"