Add IPPROTO_L2TP from Linux 6.1 to netinet/in.h

Linux 6.1 adds a define IPPROTO_L2TP to its include/uapi/linux/in.h
(not strictly a new constant, since it's moved from
include/uapi/linux/l2tp.h).  Add this constant to glibc's
netinet/in.h.

Tested for x86_64.