Updating dpkg-buildflags to enable reproducible=+fixfilepath by default

[Lisandro Damián Nicanor Pérez Meyer]

The code that relies on __FILE__ can be found at

https://sources.debian.org/src/qtbase-opensource-src/5.15.2+dfsg-2/src/testlib/qtestcase.h/#L216

So maybe we can try and get a fix there and push this change in the
next stable release, after appropriately tested.

-- 
Lisandro Damián Nicanor Pérez Meyer
http://perezmeyer.com.ar/
http://perezmeyer.blogspot.com/