Mark apt-transport-https as M-A:foreign

The package is an empty dummy by now, but even before it could have made
sense as described in the bugreport and due to external requirements it
might stick around a while still.

References: afe3cd6ef1
Closes: #905141

debian/control

@@ -160,6 +160,7 @@ Description: package management related utility programs
 
 Package: apt-transport-https
 Architecture: all
+Multi-Arch: foreign
 Depends: apt (>= 1.5~alpha4), ${misc:Depends}
 Priority: optional
 Section: oldlibs