Add missing include for VxWorks build. (#2515)

`catch_sharding.hpp` was using `std::min` without including `<algorithm>`.
This worked on most platforms, but it wasn't transitively included in
the VxWorks toolchain.

src/catch2/internal/catch_sharding.hpp

@@ -11,6 +11,7 @@
 #include <catch2/catch_session.hpp>
 
 #include <cmath>
+#include <algorithm>
 
 namespace Catch {