llama.cpp: Add a missing header for cpp23

[ykhrustalev]

The problem occurs when compiling using cpp23 due to the use of std::upper_bound.

If you are interested in adding a matrix for cpp17/23 in the CI I can do that.

• I have read the contributing guidelines
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[ggerganov]

CI is not needed - we don't officially support C++ standards later than C++11. We can accept minor compatibility changes such as this, but in general the code should be compiled with C++11.