Fix include errors with flintxx

https://github.com/wbhart/flint2/commit/c0768dc54b5ece38252857f8e7423d5ce81a43f7



diff --git a/flintxx/flint_classes.h b/flintxx/flint_classes.h
index eac2d1a..6baa392 100644
--- a/flintxx/flint_classes.h
+++ b/flintxx/flint_classes.h
@@ -27,7 +27,7 @@
 // Contrary to other parts of this library, they are tailored very
 // specifically towards FLINT.
 
-#include "flint.h"
+#include "../flint.h"
 #include "mp.h"
 #include "expression.h"
 #include "expression_traits.h"