nmap: make ndiff conditional on python2

ndiff only works when target python2 is available, so disable it
completely when it's not. Fixes:
http://autobuild.buildroot.net/results/402/40269fd96a3b62fa187e9b26e5ab4ce4e7711f95/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>