Fix host build: Add -lpthread.

Change-Id: I5c952b540d0b953b62d58b8eb191a538395c4256
diff --git a/build/Android.test.mk b/build/Android.test.mk
index ecd9d3e..5d2a225 100644
--- a/build/Android.test.mk
+++ b/build/Android.test.mk
@@ -46,6 +46,7 @@
     ART_TARGET_TEST_EXECUTABLES += $(TARGET_OUT_EXECUTABLES)/$$(LOCAL_MODULE)
   else
     ART_HOST_TEST_EXECUTABLES += $(HOST_OUT_EXECUTABLES)/$$(LOCAL_MODULE)
+    LOCAL_LDFLAGS += -lpthread
   endif
 endef