Add Longest Palindromic Substring in C++

[rzuckerm]

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Project name: Longest Palindromic Substring Project link: https://sampleprograms.io/projects/longest-palindromic-substring Language: C++

The project link must be a link to a project on the Sample Programs Website project list. If you would like to add a new project to the Sample Programs Website, please first make a pull request to the Sample Programs Website.