Build against conda-forge libnsl

[izahn]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Closes #29

I have built this package with this change locally and uploaded it to https://anaconda.org/izahn/xerces-c. I verified that this does indeed fix #29 as shown below:

$ ldd ~/mambaforge/envs/faeel/lib/libxerces-c-3.2.so | rg nsl ## the current conda-forge package
        libnsl.so.1 => /usr/lib/libnsl.so.1 (0x00007f40d64ae000)
$ ldd ~/mambaforge/envs/alela/lib/libxerces-c-3.2.so | rg nsl ## rebuilt from this PR
        libnsl.so.3 => /home/izahn/mambaforge/envs/alela/lib/./libnsl.so.3 (0x00007fdbbefd6000)

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[izahn]

@conda-forge-admin please rerender

[izahn]

I tested locally with centos8 in a docker container, and this also fixes conda-forge/rasterio-feedstock#220