libtraceevent: Add check for duplicate functions in man pages

Update the check-manpages.sh script to make sure that there are no
functions that are described in more than one man page.

The output will look like this:

  Found tep_find_function in libtraceevent-func_find.txt and in libtraceevent-func_apis.txt
  Found tep_find_function_address in libtraceevent-func_find.txt and in libtraceevent-func_apis.txt

Link: https://lore.kernel.org/linux-trace-devel/20221224153225.ojre2c3fxktfxtj2@carbon.lan/
Link: https://lore.kernel.org/linux-trace-devel/20230104172655.145516-2-rostedt@goodmis.org

Reported-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>