add arc typing to build

_CoqProject

@@ -55,6 +55,7 @@ theories/typing/lib/swap.v
 theories/typing/lib/take_mut.v
 theories/typing/lib/spawn.v
 theories/typing/lib/join.v
+theories/typing/lib/arc.v
 theories/typing/lib/mutex/mutex.v
 theories/typing/lib/mutex/mutexguard.v
 theories/typing/lib/rc/rc.v