Change end() to sen() in usertype_container.hpp

[nyllet]

i need to change this to be able to compile. i'm using g++-14 on debian linux

[DarthGandalf]

Confirmed to fix https://bugs.gentoo.org/955999 for me, thanks

[solbjorn]

I can confirm this fixes the compilation when exporting absl::btree_map as a container, thanks!