Bug#726487: frei0r build can't find opencv

[Rebecca N. Palmer]

Source: frei0r
Severity: serious
Tags: patch

frie0r's build process can't find opencv, probably because it depends on 
libcv-dev and the .pc files are now (since 2.3.1-9) in libopencv-dev.

The build succeeds as using opencv is optional, but I don't know how 
much functionality this disables (feel free to adjust the severity as 
appropriate).

It worked on my system because I do have libopencv-dev installed, which 
suggests that adding that as a build-dep would fix the problem, but I 
don't have time right now to test that properly.