rv: Fix compilation if !CONFIG_RV_REACTORS

rv: Fix compilation if !CONFIG_RV_REACTORS

The kernel test robot spotted a compilation error if reactors are
disabled.

Fix the warning by keeping LTL monitor variable as always static.

Cc: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20251113150618.185479-2-gmonaco@redhat.com
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511131948.vxi5mdjU-lkp@intel.com/
Fixes: 4f739ed19d22 ("rv: Pass va_list to reactors")
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>