fnott bundles nanosvg in-tree and builds with it, which is licensed
under the zlib license. The `license` meta-attr should reflect this (as
this more or less means nanosvg is part of the project), thus add the
zlib license to it.
See the pull-request discussion [0] and other precedents like [1].
[0] https://github.com/NixOS/nixpkgs/pull/225224
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=982145#74
Signed-off-by: Christoph Heiss <christoph@c8h4.io>