/usr/include/lv2/lv2plug.in/ns/ is in lv2-dev 1.0.0~dfsg2-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/lv2/lv2plug.in/ns/ext/
/usr/include/lv2/lv2plug.in/ns/ext/atom
/usr/include/lv2/lv2plug.in/ns/ext/data-access
/usr/include/lv2/lv2plug.in/ns/ext/dynmanifest
/usr/include/lv2/lv2plug.in/ns/ext/event
/usr/include/lv2/lv2plug.in/ns/ext/instance-access
/usr/include/lv2/lv2plug.in/ns/ext/log
/usr/include/lv2/lv2plug.in/ns/ext/midi
/usr/include/lv2/lv2plug.in/ns/ext/patch
/usr/include/lv2/lv2plug.in/ns/ext/port-groups
/usr/include/lv2/lv2plug.in/ns/ext/port-props
/usr/include/lv2/lv2plug.in/ns/ext/presets
/usr/include/lv2/lv2plug.in/ns/ext/resize-port
/usr/include/lv2/lv2plug.in/ns/ext/state
/usr/include/lv2/lv2plug.in/ns/ext/time
/usr/include/lv2/lv2plug.in/ns/ext/uri-map
/usr/include/lv2/lv2plug.in/ns/ext/urid
/usr/include/lv2/lv2plug.in/ns/ext/worker
/usr/include/lv2/lv2plug.in/ns/extensions/
/usr/include/lv2/lv2plug.in/ns/extensions/ui
/usr/include/lv2/lv2plug.in/ns/extensions/units
/usr/include/lv2/lv2plug.in/ns/lv2core