/usr/include/syslog-ng/logproto/ is in syslog-ng-dev 3.5.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/syslog-ng/logproto/logproto-buffered-server.h
/usr/include/syslog-ng/logproto/logproto-builtins.h
/usr/include/syslog-ng/logproto/logproto-client.h
/usr/include/syslog-ng/logproto/logproto-dgram-server.h
/usr/include/syslog-ng/logproto/logproto-framed-client.h
/usr/include/syslog-ng/logproto/logproto-framed-server.h
/usr/include/syslog-ng/logproto/logproto-indented-multiline-server.h
/usr/include/syslog-ng/logproto/logproto-record-server.h
/usr/include/syslog-ng/logproto/logproto-regexp-multiline-server.h
/usr/include/syslog-ng/logproto/logproto-server.h
/usr/include/syslog-ng/logproto/logproto-text-client.h
/usr/include/syslog-ng/logproto/logproto-text-server.h
/usr/include/syslog-ng/logproto/logproto.h