add libvtk6-qt-dev dependency to build on Debian Stretch

[mikaelarguedas]

the libpcl-dev package is missing a dependency causing the Qt5::Widgets target to be exported but not satisfied.

This should fix the currently failing Stretch builds (e.g. http://build.ros.org/job/Lbin_ds_dS64__simple_grasping__debian_stretch_amd64__binary/1/)

[mikeferguson]

@mikaelarguedas -- it seems this prevents me from releasing into Lunar at all - -guess we need to update rosdep keys:

Releasing for rosdistro: lunar

Pre-verifying Debian dependency keys...
Running 'rosdep update'...
Could not resolve rosdep key 'libvtk6-qt-dev'
Failed to resolve libvtk6-qt-dev on ubuntu:xenial with: Error running generator: Failed to resolve rosdep key 'libvtk6-qt-dev', aborting.
libvtk6-qt-dev is depended on by these packages: ['simple_grasping']
<== Failed
Some of the dependencies for packages in this repository could not be resolved by rosdep.
You can try to address the issues which appear above and try again if you wish.

[mikeferguson]

Actually it seems the correct key is libvtk-qt -- I'm gonna fix that now

[mikaelarguedas]

>_< I Used the ubuntu package name and not the rosdep key, the rosdep key looks to be libvtk-qt sorry about that

[mikaelarguedas]

also just noticed there was a typo in my patch: "Needed yo work" -> "Needed to work"

[mikeferguson]

Yup -- both are fixed in 5ce2dfe

[mikaelarguedas]

🎉