Fixed build with kernels 6.4.10+
Gentoo Bug: https://bugs.gentoo.org/912242
See also:   https://github.com/mtorromeo/r8168/issues/54

---
--- a/src/r8168_n.c     2023-08-13 03:33:06.977422132 +0400
+++ b/src/r8168_n.c     2023-08-13 03:38:39.767005101 +0400
@@ -81,6 +81,10 @@
 #include <linux/mdio.h>
 #endif

+#if LINUX_VERSION_CODE >= KERNEL_VERSION(6,4,10)
+#include <net/gso.h>
+#endif
+
 #include <asm/io.h>
 #include <asm/irq.h>