aapt: expat exports its header files now.

Change-Id: Iddf0133919d38fc896f97c6796f8228a6decf174
diff --git a/tools/aapt2/XmlDom.h b/tools/aapt2/XmlDom.h
index 6931884..035e7c4 100644
--- a/tools/aapt2/XmlDom.h
+++ b/tools/aapt2/XmlDom.h
@@ -21,7 +21,7 @@
 #include "StringPiece.h"
 
 #include <istream>
-#include <libexpat/expat.h>
+#include <expat.h>
 #include <memory>
 #include <string>
 #include <vector>