build system: fix a few warnings for allnoconfig build

Not that allnoconfig build is useful in any way...

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

applets/usage_pod.c

@@ -90,6 +90,8 @@ int main(void)
 				printf("%s\n\n", usage_array[i].aname);
 		}
 	}
+	printf("=back\n\n");
+
 	return 0;
 }