Add ubuntu-raspi-settings to PPU

[Dave Jones]

Dear DMB,

Just before mantic's release we split the ubuntu-raspi-settings binary 
package out of the ubuntu-settings source package into its own source 
package (enabling a last-minute fix to the raspi images without 
requiring a re-spin of all the other packages affected by 
ubuntu-settings).

Given ubuntu-raspi-settings is raspi-specific, could it be added to my 
raspi-specific PPU?

Best regards,

Dave Jones.