Fix build - needed to refer to libufs.json file by name

Signed-off-by: Graham MacDonald <grahamamacdonald@gmail.com>

sys/src/libs.json

@@ -32,7 +32,7 @@
 			"/sys/src/libsec/libsec.json",
 			"/sys/src/libstdio/libstdio.json",
 			"/sys/src/libthread/libthread.json",
-			"/sys/src/libufs",
+			"/sys/src/libufs/libufs.json",
 			"/sys/src/libusb",
 			"/sys/src/libventi/libventi.json"
 		]