tracing/hist: add in missing * in comment blocks

There are a couple of missing * in comment blocks. Fix these.
Cleans up two clang warnings:

kernel/trace/trace_events_hist.c:986: warning: bad line:
kernel/trace/trace_events_hist.c:3229: warning: bad line:

Link: https://lkml.kernel.org/r/20221020133019.1547587-1-colin.i.king@gmail.com

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -983,7 +983,7 @@ static struct hist_field *find_any_var_ref(struct hist_trigger_data *hist_data,
  * A trigger can define one or more variables.  If any one of them is
  * currently referenced by any other trigger, this function will
  * determine that.
-
+ *
  * Typically used to determine whether or not a trigger can be removed
  * - if there are any references to a trigger's variables, it cannot.
  *
@@ -3226,7 +3226,7 @@ static struct field_var *create_field_var(struct hist_trigger_data *hist_data,
  * events.  However, for convenience, users are allowed to directly
  * specify an event field in an action, which will be automatically
  * converted into a variable on their behalf.
-
+ *
  * This function creates a field variable with the name var_name on
  * the hist trigger currently being defined on the target event.  If
  * subsys_name and event_name are specified, this function simply