subsys/bluetooth/host/keys.h: Add include guard and required include

This header requires bluetooth.h but was not including it itself.
Due to this we had 2 tests failing to build.
Let's just include the dependencies to this header instead of
relaying on users including the dependencies dependencies in
the right order.

Also, let's add an include guard as in a test this header was
included twice leading to weird build errors.

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

subsys/bluetooth/host/keys.h

@@ -6,6 +6,11 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#ifndef ZEPHYR_SUBSYS_BLUETOOTH_HOST_KEYS_H_
+#define ZEPHYR_SUBSYS_BLUETOOTH_HOST_KEYS_H_
+
+#include <zephyr/bluetooth/bluetooth.h>
+
 /** @cond INTERNAL_HIDDEN */
 
 enum bt_keys_type {
@@ -233,3 +238,5 @@ void bt_keys_link_key_update_usage(const bt_addr_t *addr);
 void bt_keys_show_sniffer_info(struct bt_keys *keys, void *data);
 
 /** @endcond */
+
+#endif /* ZEPHYR_SUBSYS_BLUETOOTH_HOST_KEYS_H_ */