/usr/include/xmlsec1/ is in libxmlsec1-dev 1.2.18-2ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/xmlsec1/xmlsec/
/usr/include/xmlsec1/xmlsec/app.h
/usr/include/xmlsec1/xmlsec/base64.h
/usr/include/xmlsec1/xmlsec/bn.h
/usr/include/xmlsec1/xmlsec/buffer.h
/usr/include/xmlsec1/xmlsec/crypto.h
/usr/include/xmlsec1/xmlsec/dl.h
/usr/include/xmlsec1/xmlsec/errors.h
/usr/include/xmlsec1/xmlsec/exports.h
/usr/include/xmlsec1/xmlsec/gcrypt/
/usr/include/xmlsec1/xmlsec/gcrypt/app.h
/usr/include/xmlsec1/xmlsec/gcrypt/crypto.h
/usr/include/xmlsec1/xmlsec/gcrypt/symbols.h
/usr/include/xmlsec1/xmlsec/gnutls/
/usr/include/xmlsec1/xmlsec/gnutls/app.h
/usr/include/xmlsec1/xmlsec/gnutls/crypto.h
/usr/include/xmlsec1/xmlsec/gnutls/symbols.h
/usr/include/xmlsec1/xmlsec/gnutls/x509.h
/usr/include/xmlsec1/xmlsec/io.h
/usr/include/xmlsec1/xmlsec/keyinfo.h
/usr/include/xmlsec1/xmlsec/keys.h
/usr/include/xmlsec1/xmlsec/keysdata.h
/usr/include/xmlsec1/xmlsec/keysmngr.h
/usr/include/xmlsec1/xmlsec/list.h
/usr/include/xmlsec1/xmlsec/membuf.h
/usr/include/xmlsec1/xmlsec/nodeset.h
/usr/include/xmlsec1/xmlsec/nss/
/usr/include/xmlsec1/xmlsec/nss/app.h
/usr/include/xmlsec1/xmlsec/nss/bignum.h
/usr/include/xmlsec1/xmlsec/nss/crypto.h
/usr/include/xmlsec1/xmlsec/nss/keysstore.h
/usr/include/xmlsec1/xmlsec/nss/pkikeys.h
/usr/include/xmlsec1/xmlsec/nss/symbols.h
/usr/include/xmlsec1/xmlsec/nss/x509.h
/usr/include/xmlsec1/xmlsec/openssl/
/usr/include/xmlsec1/xmlsec/openssl/app.h
/usr/include/xmlsec1/xmlsec/openssl/bn.h
/usr/include/xmlsec1/xmlsec/openssl/crypto.h
/usr/include/xmlsec1/xmlsec/openssl/evp.h
/usr/include/xmlsec1/xmlsec/openssl/symbols.h
/usr/include/xmlsec1/xmlsec/openssl/x509.h
/usr/include/xmlsec1/xmlsec/parser.h
/usr/include/xmlsec1/xmlsec/private/
/usr/include/xmlsec1/xmlsec/private.h
/usr/include/xmlsec1/xmlsec/private/xkms.h
/usr/include/xmlsec1/xmlsec/private/xslt.h
/usr/include/xmlsec1/xmlsec/soap.h
/usr/include/xmlsec1/xmlsec/strings.h
/usr/include/xmlsec1/xmlsec/templates.h
/usr/include/xmlsec1/xmlsec/transforms.h
/usr/include/xmlsec1/xmlsec/version.h
/usr/include/xmlsec1/xmlsec/x509.h
/usr/include/xmlsec1/xmlsec/xkms.h
/usr/include/xmlsec1/xmlsec/xmldsig.h
/usr/include/xmlsec1/xmlsec/xmlenc.h
/usr/include/xmlsec1/xmlsec/xmlsec.h
/usr/include/xmlsec1/xmlsec/xmltree.h