IPPROTO_L2TP was moved from linux/l2tp.h to linux/in.h [0], so let's
reflect that change to fix build with newer kernels:
```
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:10:
../src/basic/linux/l2tp.h:16: error: "IPPROTO_L2TP" redefined [-Werror]
16 | #define IPPROTO_L2TP 115
|
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:3:
/usr/include/netinet/in.h:85: note: this is the location of the previous definition
85 | #define IPPROTO_L2TP IPPROTO_L2TP
|
cc1: all warnings being treated as errors
```
When at it, update the rest of the headers we ship as well.
[0] https://github.com/torvalds/linux/commit/65b32f801bfbc54dc98144a6ec26082b59d131ee
