utils: Steal `steal` from libei
Add excerpt of `util-mem.h` from libei defining the macro `steal`, in
order to improve memory management and the code semantics.
See: https://gitlab.freedesktop.org/libinput/libei/-/blob/38132d6fc5905e2d4361325fdded29a0dff990d6/src/util-mem.h#L92