build_patches: reintroduce updated l2tp stats patch

Like in the earlier patch, this suppresses the integer overflow
warning for l2tp on 32 bit systems. However, like before, the
underlying counter variable is still a 32-bit integer, so the
traffic counters will just restart at zero after reaching
2^32 bytes = 4 GiB.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>
Reviewed-by: Dominik Heidler <dominik@heidler.eu>