x/sys/unix: update Dockerfile to Kernel 5.13

[lhl2617]

Similar to CL 313391 (Kernel 5.12, Go 1.16.3)

[lhl2617]

There seems to be a lot of definition/enumeration conflicts stemming from <netinet/in.h>.

Running

GOOS=linux GOARCH=amd64 ./mkall.sh

after changing the kernel git clone line in the Dockerfile to 5.13 gives:
https://gist.github.com/lhl2617/4f4407b5549376bc26946b247e8ddd82

[gopherbot]

Change https://golang.org/cl/332929 mentions this issue: unix: move ICMP{,V6}_FILTER* const definitions

[lhl2617]

For reference, a non-ideal, sledgehammer approach: https://www.linuxquestions.org/questions/linux-software-2/conflict-between-kernel-and-glibc-headers-when-compiling-ppp-4175594417/

[gopherbot]

Change https://golang.org/cl/332969 mentions this issue: unix/linux: update Dockerfile to Kernel 5.13