Fix compile err in example dir

[iysheng]

When I build in example dir on fedora 40 server. I get some errors like below:

▸ make
...                                                                                                                                                                                                                                                                                                                             make: *** No rule to make target 'issue', needed by 'all'.  Stop.
make: *** Waiting for unfinished jobs....

...

▸ make clean
rm server client hello simplecli simplesvr upload redirect ssesvr ssecli benchmark one_time_request server_and_client *.pem                                                                                                                           rm: cannot remove 'one_time_request': No such file or directory
rm: cannot remove 'server_and_client': No such file or directory
rm: cannot remove '*.pem': No such file or directory

So I made this modify to optimize makefile in example dir.

[yhirose]

@iysheng thanks for the pull request. (I left '*.pem', since I need it once I execute make pem.