getconf: Add an explicit return at the end of main

ISO C does not require that main() have an explicit return value
(if the end of the main function is reached, it is as if it returned
0). However, when built as sbase-box, the main function is turned
into getconf_main, which does not have the same special treatment.

getconf.c

@@ -103,4 +103,6 @@ main(int argc, char *argv[])
 	}
 
 	eprintf("invalid variable: %s\n", argv[0]);
+
+	return 0;
 }