scripts: add const_structs.checkpatch for checkpatch.pl

scripts: add const_structs.checkpatch for checkpatch.pl

Kernel has separated the structs that are reported to be const in
checkpatch.pl into a file of its own, const_structs.checkpatch.

This file has been missing after the recent update of checkpatch.pl,
leading to the following message:

  No structs that should be const will be found - file
  '/data/openwrt/scripts/const_structs.checkpatch': No such file
  or directory

This commit adds the relevant file from v5.10-rc4.

Fixes: 086ee09bbcac ("scripts: Update checkpatch.pl to 2020-06-11")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>