radix tree test suite: Fix compilation

An include of xarray.h was added to lib/idr.c without updating the test
suite.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

tools/testing/radix-tree/linux/xarray.h

@@ -0,0 +1,2 @@
+#include "generated/map-shift.h"
+#include "../../../../include/linux/xarray.h"