Bug#852447: qtbase-opensource-src: Don't build docs when nodoc profile is enabled

[Александр Волков <a.volkov@rusbitech.ru>]

Package: qtbase-opensource-src
Version: 5.7.1+dfsg-3

nodoc profile is more clear than Build-Depends-Indep if you want to 
boostrap.
It would be nice to use it for other Qt packages too.