tree-wide: Make sure all headers are standalone

Let's make sure all of our headers include everything they require.

src/boot/url-discovery.h

@@ -1,4 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include "efi.h"
+
 char16_t *disk_get_url(EFI_HANDLE *handle);