readers: define query::partition_slice before using it in default argument

C++23 made std::unique_ptr constexpr. A side effect of this (presumably)
is that the compiler compiles it more eagerly, requiring the full definition
of the class in std::make_unique, while it previously was content with
finding the definition later.

One victim of this change is the default argument of make_reversing_reader;
define it earlier (by including its header) to build with C++23.

readers/reversing_v2.hh

@@ -8,12 +8,12 @@
 
 #pragma once
 #include <memory>
+#include "query-request.hh"
 
 class mutation_reader;
 
 namespace query {
     struct max_result_size;
-    class partition_slice;
 }