Added qtdeclarative5-dev to debian control file to fix ppa builds

[daschuer]

This should fix our nightly PPA

[Holzhaus]

Thanks, does it need to be added to the cpack config as well?

[daschuer]

For my understanding the debian tools take automatically care of that.
We will see.

[Be-ing]

I'll merge this now. If there's more to be done, it can be done in another PR.

[Be-ing]

PPA build failed:

 dpkg-buildpackage -rfakeroot -us -uc -ui -S -sa
dpkg-buildpackage: info: source package mixxx
dpkg-buildpackage: info: source version 2.4.0~alpha~pre-0ubuntu1~master~git7429~bionic
dpkg-buildpackage: info: source distribution bionic
dpkg-buildpackage: info: source changed by RJ Ryan <rryan@mixxx.org>
 dpkg-source --before-build mixxx-2.4.0-alpha-pre
dpkg-source: warning: can't parse dependency qtdeclarative5-dev
              qtscript5-dev
dpkg-source: error: error occurred while parsing Build-Depends
dpkg-buildpackage: error: dpkg-source --before-build mixxx-2.4.0-alpha-pre subprocess returned exit status 25
debuild: fatal error at line 1152:
dpkg-buildpackage -rfakeroot -us -uc -ui -S -sa failed
* Build failed.

https://builds.renegadetech.mixxx.org/job/master-nightly-ppa/666/console