Fix trivial compile issue

Change-Id: I9dbc51496dbdb2195bbe589b0c3b2cd7e3d2f1fc
diff --git a/native/src/proximity_info.h b/native/src/proximity_info.h
index 35e354c..19b84cc 100644
--- a/native/src/proximity_info.h
+++ b/native/src/proximity_info.h
@@ -21,6 +21,10 @@
 
 #include "defines.h"
 
+#ifndef NULL
+#define NULL 0
+#endif
+
 namespace latinime {
 
 class Correction;