ART: Fix missing role definition

Add missing static member definition. Follow-up to
4e2cb098017bf073335ebb02b1bc0a36828cd720

Bug: 20072211
Change-Id: I392da3724ded7c455b9b57add6fceb70191f9ff6
diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index c591a51..62cfb52 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -62,6 +62,7 @@
 Mutex* Locks::trace_lock_ = nullptr;
 Mutex* Locks::unexpected_signal_lock_ = nullptr;
 Mutex* Locks::lambda_table_lock_ = nullptr;
+Uninterruptible Roles::uninterruptible_;
 
 struct AllMutexData {
   // A guard for all_mutexes_ that's not a mutex (Mutexes must CAS to acquire and busy wait).