aapt: expat exports its header files now.

Change-Id: Iddf0133919d38fc896f97c6796f8228a6decf174
diff --git a/tools/aapt/XMLNode.h b/tools/aapt/XMLNode.h
index 3161f65..b9e5cd5 100644
--- a/tools/aapt/XMLNode.h
+++ b/tools/aapt/XMLNode.h
@@ -10,6 +10,8 @@
 #include "StringPool.h"
 #include "ResourceTable.h"
 
+#include <expat.h>
+
 class XMLNode;
 
 extern const char* const RESOURCES_ROOT_NAMESPACE;