fix std::fmin compilation issue
fix std::fmin compilation issue

When building with msan I get compiler errors about std::fmin, including
<cmath> fixes it. Likely this is platform dependent since I don't get it
without msan, and the continuous integration doesn't seem to get the
error with msan either.

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl