Update diy2 to fix some Android builds

On some API levels, the compilation of diy2 used to fail
because of call to mkostemp. It was replaced by mkstemp.

ThirdParty/diy2/update.sh

@@ -8,7 +8,7 @@ readonly name="diy2"
 readonly ownership="Diy2 Upstream <kwrobot@kitware.com>"
 readonly subtree="ThirdParty/$name/vtk$name"
 readonly repo="https://gitlab.kitware.com/third-party/diy2.git"
-readonly tag="for/vtk-20191127-gd6d04d0d"
+readonly tag="for/vtk-20210721-gd6d04d0d"
 readonly paths="
 .gitattributes
 CMakeLists.vtk.txt