Bump extern/yaml-cpp from c2bec4c to 9ce5a25 (#800)

Bumps [extern/yaml-cpp](https://github.com/jbeder/yaml-cpp) from
`c2bec4c` to `9ce5a25`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/9ce5a25188d83b43dd5cd633f2975be10f5d6608"><code>9ce5a25</code></a>
fix: spell fix</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/ebfbf2711582bd6a35198e88df0820ad3b5d3cc2"><code>ebfbf27</code></a>
patch: added more defensive programming techniques</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/06ffaf3104cbae3b206fdf12f70888e442e5ebcc"><code>06ffaf3</code></a>
remove reduntant license information</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/0b945415fd7819c732ad9de7ff1b1c851ba364f8"><code>0b94541</code></a>
fix: add missing include</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/82f3c04081af289b51515fb4936268cbd4ab3fc8"><code>82f3c04</code></a>
fix: add YAML_CPP_API for windows-shared library builds</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/d8de96524cf62ba3b70111d997510aae700958b0"><code>d8de965</code></a>
patch: move dragonbox.h from 'include' folder to 'src' folder</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/5d9e4b6251b3cfc9fff41a8d980c1a3bfdfaf019"><code>5d9e4b6</code></a>
patch: split fp_to_string.h into fptostring.h and fptostring.cpp</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/28c0a1bc25ea1d241ebc8a3e3c9f16dde030fbf5"><code>28c0a1b</code></a>
doc: add a little more information to dragonbox file</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/72865f7d328978c67cd4fa151a4678b1a6d3fcbc"><code>72865f7</code></a>
fix: remove trailing return types</li>
<li><a
href="https://github.com/jbeder/yaml-cpp/commit/f4bca79dc40fcc6a0334eb6a848d7e77d37df682"><code>f4bca79</code></a>
fix: fp_to_string uses now 2 spaces</li>
<li>Additional commits viewable in <a
href="https://github.com/jbeder/yaml-cpp/compare/c2bec4c755c67ad86185a2a264996137904fb712...9ce5a25188d83b43dd5cd633f2975be10f5d6608">compare
view</a></li>
</ul>
</details>
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