Fix _socket compilation failures on non-Linux buildbots (cf. issue #1746656).
diff --git a/Modules/socketmodule.h b/Modules/socketmodule.h
index f064795..db44fd3 100644
--- a/Modules/socketmodule.h
+++ b/Modules/socketmodule.h
@@ -59,9 +59,12 @@
 #include <bluetooth.h>
 #endif
 
+#ifdef HAVE_NET_IF_H
+# include <net/if.h>
+#endif
+
 #ifdef HAVE_NETPACKET_PACKET_H
 # include <sys/ioctl.h>
-# include <net/if.h>
 # include <netpacket/packet.h>
 #endif