For me, the following seems to work:
ln -sf /usr/lib/libfontconfig.so.1 /usr/lib/insync/libfontconfig.so.1
This will overwrite the libfontconfig.so bundled with insync and replace it with a symlink to the system library; so if you want to keep the insync one, rename it first.
To proceed more cleanly, you could also modify the insync PKGBUILD to include
ln -sf "/usr/lib/libfontconfig.so.1" "${pkgdir}/usr/lib/insync/libfontconfig.so.1"
in its package() function and then rebuild and reinstall the package.