verification/rvgen: Generate each variable definition only once

If a variable appears multiple times in the specification, ltl2k generates
multiple variable definitions. This fails the build.

Make sure each variable is only defined once.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/107dcf0d0aa8482d5fbe0314c3138f61cd284e91.1752850449.git.namcao@linutronix.de


Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>