bt: cert: declare missing types in headers

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

Author: mniestroj

include/pouch_gateway/bt/cert.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+struct bt_conn;
+
 /**
  * Start certificate exchange for the given Bluetooth connection.
  *

include/pouch_gateway/bt/downlink.h

@@ -6,6 +6,9 @@
 
 #pragma once
 
+struct bt_conn;
+struct pouch_gateway_downlink_context;
+
 /**
  * Start downlink for the given Bluetooth connection.
  *

include/pouch_gateway/bt/uplink.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+struct bt_conn;
+
 /**
  * Start uplink for the given Bluetooth connection.
  *