jinja2cpp: update submodule for 'not X is defined' fix

[cebtenzzre]

It turns out that the reason #3372 was needed was that if not add_generation_prompt is defined was meant to be parsed as if not (add_generation_prompt is defined), which is how it works in Jinja2Cpp and also how those operators work in the Python language itself ('is' binds more strongly than 'not').

But Jinja2Cpp parsed that as if (not add_generation_prompt) is defined, which when read that way is nonsense—it will always be true, so the conditional will always be executed.

With this change, the operator precedence is corrected, so Jinja2Cpp is able to interpret these expressions in the expected manner. This leaves list slicing as the primary limitation preventing Jinja2Cpp from being compatible with most prompt templates.