libtracefs: Rename tracefs_hist_reset_sort_key() tracefs_hist_set_sort_key()

The commit log that added tracefs_hist_reset_sort_key() called it
tracefs_hist_set_sort_key(), and that seems to be a better name.

Link: https://lore.kernel.org/linux-trace-devel/20211110231025.1131051-3-rostedt@goodmis.org

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>