/usr/lib/x86_64-linux-gnu/pkgconfig/libpng.pc is in libpng12-dev 1.2.54-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/pkgconfig/libpng12.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/libpng12.pc