debian/control: Bump required mit-scheme to 9.2.2.

debian/control: Bump required mit-scheme to 9.2.2.

The new version includes microcode-id/operating-system-suffix which is
used by plugin Makefile.ams.  Plugin binaries should be built by the
pucked core they will run in but, with this support, their source
distributions may be built by mit-scheme.  Plugin Makefile.ams also
expect mit-scheme.h to be installed, but the Debian packages for 9.2.1
did not include it.  9.2.2-1 should fix that.