libtracefs: Have tracefs_sql() cast handle stacktrace

A histogram can have a "stacktrace" type. Allow casts to make that:

  # sqlhist select cast '('start.stack as stacktrace')' from sleep_lat as start
  echo 'hist:keys=stack.stacktrace' > /sys/kernel/tracing/events/synthetic/sleep_lat/trigger

Link: https://lore.kernel.org/20250410212112.45e673ad@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>