Fix build of www/otter-browser

The qmake port is required to be installed when compiling otter. While this
requirement is usually met due to its dependencies on qt5 elements which
depend on qmake, in case someone uninstalled qmake which is not a runtime
dependency for any qt5* port, otter would not build any more.

Resolve this introducing a dependency on qmake.

Reported by a build cluster, log:
http://beefy2.isc.freebsd.org/data/10amd64-default/2014-12-17_04h21m21s/logs/otter-browser-0.9.03.log

Approved by: rakuco (mentor)