Add a Breaks on flatpak (<< 0.8.7-2~)

Those versions rely on libostree to download the summary and its
signature when mirroring. libostree >= 2017.7 no longer does this. On
affected flatpak versions, this breaks installation of new apps and
runtimes system-wide.

debian/control

@@ -68,6 +68,8 @@ Depends:
  ${shlibs:Depends},
 Pre-Depends:
  ${misc:Pre-Depends},
+Breaks:
+ flatpak (<< 0.8.7-2~),
 Multi-arch: same
 Description: content-addressed filesystem for operating system binaries (library)
  OSTree is a tool and library for managing bootable, immutable, versioned