Adding libssl-dev as dependency in README.md

## Description v4l2rtspserver doesn't make without libssl-dev files ## Related Issue Not Available ## Motivation and Context OpenSSL dev files are required to compile this project properly. ## Types of changes - [x] Docs change / refactoring / dependency upgrade - [ ] Bug fix (non-breaking change which fixes an issue) - [ ] New feature (non-breaking change which adds functionality) - [ ] Breaking change (fix or feature that would cause existing functionality to change)