Add missing group field for cli argp_options

Rebuilding, we get the following error:

```
missing initializer for field 'group' of 'const struct argp_option'
[-Werror=missing-field-initializers]
```

argp_option (from argp.h) has one more field 'group', denoting the cli
option grouping when displaying groups of options to users.  Adding
group ids to avoid the compilation warning/error.

linkevents.c

@@ -38,8 +38,8 @@ const char argp_program_doc[] =
 "    linkevents -v\n";
 
 static const struct argp_option opts[] = {
-	{ "verbose", 'v', NULL, 0, "Verbose debug output" },
-	{ NULL, 'h', NULL, OPTION_HIDDEN, "Show the full help" },
+	{ "verbose", 'v', NULL, 0, "Verbose debug output", 0 },
+	{ NULL, 'h', NULL, OPTION_HIDDEN, "Show the full help", -1 },
 	{},
 };