ping: Make ping_rts struct static

This allows accessing global_rts->exiting in sigexit() signal handler
after main() has exited. Problem occurred on aarch64, which occasionally
delivered signal after main() has exited, which causes segfault.

Fixes: b3a41a6e ("ping: move global variables to runtime config structure")
Fixes: https://github.com/iputils/iputils/issues/423
Closes: https://github.com/iputils/iputils/pull/425



Reported-by: linzhanglong
Suggested-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Petr Vorel <pvorel@suse.cz>