Try to fix sim build.  Looks like gettid() is in unistd.h.

Change-Id: Ib1ecea86246ad75b2553b0ccc8ce03a53ffdf218
diff --git a/core/jni/android_util_Binder.cpp b/core/jni/android_util_Binder.cpp
index ea90b59..2fd68b8 100644
--- a/core/jni/android_util_Binder.cpp
+++ b/core/jni/android_util_Binder.cpp
@@ -24,6 +24,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <stdio.h>
+#include <unistd.h>
 
 #include <utils/Atomic.h>
 #include <binder/IInterface.h>