/usr/lib/x86_64-linux-gnu/libcurl.a is in libcurl4-nss-dev 7.52.1-5+deb9u9.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libcurl-nss.a
/usr/lib/x86_64-linux-gnu/libcurl-nss.a