tty: Partially revert the removal of the Cyclades public API

Fix a user API regression introduced with commit f76edd8f ("tty:
cyclades, remove this orphan"), which removed a part of the API and
caused compilation errors for user programs using said part, such as
GCC 9 in its libsanitizer component[1]:

.../libsanitizer/sanitizer_common/sanitizer_platform_limits_posix.cc:160:10: fatal error: linux/cyclades.h: No such file or directory
  160 | #include <linux/cyclades.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[4]: *** [Makefile:664: sanitizer_platform_limits_posix.lo] Error 1

As the absolute minimum required bring `struct cyclades_monitor' and
ioctl numbers back then so as to make the library build again.  Add a
preprocessor warning as to the obsolescence of the features provided.

References:

[1] GCC PR sanitizer/100379, "cyclades.h is removed from linux kernel
    header files", <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100379>

Fixes: f76edd8f ("tty: cyclades, remov...