WireGuardKit: Set publicHeadersPath = "." to flatten public headers structure

Signed-off-by: Andrej Mihajlov <and@mullvad.net>

Sources/WireGuardKitC/WireGuardKitC.h

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
 // Copyright © 2018-2019 WireGuard LLC. All Rights Reserved.
 
-#include "../key.h"
-#include "../x25519.h"
+#include "key.h"
+#include "x25519.h"

Sources/WireGuardKitC/module.modulemap

@@ -1,4 +1,4 @@
 module WireGuardKitC {
-    umbrella header "include/WireGuardKitCTarget.h"
+    umbrella header "WireGuardKitC.h"
     export *
 }