Fix the Build-Depends syntax in d/control

The deb822 format does not seem to allow for leading commas in lists.

This caused the CI to fail with:

+ apt-get -qy -o APT::Get::Build-Dep-Automatic=yes -o Acquire::http::Pipeline-Depth=0 build-dep /builds/pkg/fstrm
Note, using directory '/builds/pkg/fstrm' to get the build dependencies
Reading package lists...
E: Problem parsing dependency: Build-Depends

Remove the leading command to let apt parse the Build-Depends field.