cli: use an error quark for domain in g_set_error() instead of 0 (rh #842975)

glib 2.32 makes a runtime check that domain is not NULL:
GLib-WARNING **: (gerror.c:390):g_error_new_valist: runtime check failed: (domain != 0)

cli/src/nmcli.h

@@ -119,4 +119,8 @@ typedef struct _NmCli {
 	gboolean nocheck_ver;                             /* Don't check nmcli and NM versions: option '--nocheck' */
 } NmCli;
 
+/* Error quark for GError domain */
+#define NMCLI_ERROR (nmcli_error_quark ())
+GQuark nmcli_error_quark (void);
+
 #endif /* NMC_NMCLI_H */