Remove unnecessary "sys/" from errno.h include.

Looks like it's not needed, and most systems' manpages say to just
include <errno.h>.

examples/shell/shell_common/cmd_ping.cpp

@@ -17,7 +17,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-#include <sys/errno.h>
+#include <errno.h>
 
 #include <core/CHIPCore.h>
 #include <messaging/ExchangeMgr.h>