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: NMatthew Wilcox <willy@infradead.org>

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

+#include "generated/map-shift.h"
+#include "../../../../include/linux/xarray.h"