include: net/if.h requires features.h

The header uses __THROW, which is defined in features.h, to make the
header self-consistent.
Note, src/basic/include/sys/mount.h also uses __THROW, and includes
features.h.

src/basic/include/net/if.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <features.h>
 #include <linux/if.h>   /* IWYU pragma: export */
 
 #define IF_NAMESIZE       16