tty: Partially revert the removal of the Cyclades public API

Fix a user API regression introduced with commit f76edd8 ("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>

Signed-off-by: Maciej W. Rozycki <macro@embecosm.com>
Fixes: f76edd8 ("tty: cyclades, remove this orphan")
Cc: stable@vger.kernel.org # v5.13+

include/uapi/linux/cyclades.h

@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+
+#ifndef _UAPI_LINUX_CYCLADES_H
+#define _UAPI_LINUX_CYCLADES_H
+
+#warning "Support for features provided by this header has been removed"
+#warning "Please consider updating your code"
+
+struct cyclades_monitor {
+	unsigned long int_count;
+	unsigned long char_count;
+	unsigned long char_max;
+	unsigned long char_last;
+};
+
+#define CYGETMON		0x435901
+#define CYGETTHRESH		0x435902
+#define CYSETTHRESH		0x435903
+#define CYGETDEFTHRESH		0x435904
+#define CYSETDEFTHRESH		0x435905
+#define CYGETTIMEOUT		0x435906
+#define CYSETTIMEOUT		0x435907
+#define CYGETDEFTIMEOUT		0x435908
+#define CYSETDEFTIMEOUT		0x435909
+#define CYSETRFLOW		0x43590a
+#define CYGETRFLOW		0x43590b
+#define CYSETRTSDTR_INV		0x43590c
+#define CYGETRTSDTR_INV		0x43590d
+#define CYZSETPOLLCYCLE		0x43590e
+#define CYZGETPOLLCYCLE		0x43590f
+#define CYGETCD1400VER		0x435910
+#define CYSETWAIT		0x435912
+#define CYGETWAIT		0x435913
+
+#endif /* _UAPI_LINUX_CYCLADES_H */