Update dependencies.txt

adding missing dependency
python-nmap is needed for rscan.py

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Bywaf> use scan/rscan.py
Loading path: '/home/kour/PycharmProjects/OWASP-ByWaf/scan/rscan.py'                                                                 
Error: can't import module '/home/kour/PycharmProjects/OWASP-ByWaf/scan/rscan.py'; 'No module named nmap'