Add missoncontrolpath variable to mission-control-plugins.pc
Add missoncontrolpath variable to mission-control-plugins.pc

This is for testing so plugins can lookup where the mission control
executable is.

https://bugs.freedesktop.org/show_bug.cgi?id=35140

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control