Fix LLVM build.

Change-Id: Id49a1c13fa44cb1d7638cb8601fbf55b2f3770d6
diff --git a/src/compiler/Frontend.cc b/src/compiler/Frontend.cc
index 72eb8a1..9db23ac 100644
--- a/src/compiler/Frontend.cc
+++ b/src/compiler/Frontend.cc
@@ -25,7 +25,9 @@
 #include <llvm/Support/Threading.h>
 
 namespace {
+#if !defined(ART_USE_LLVM_COMPILER)
   pthread_once_t llvm_multi_init = PTHREAD_ONCE_INIT;
+#endif
   void InitializeLLVMForQuick() {
     llvm::llvm_start_multithreaded();
   }