cpp-btree: Remove remaining uses of ssize_t

This is due to issues building on Windows

src/3rdparty/cpp-btree/btree.h

@@ -886,7 +886,7 @@ class btree : public Params::key_compare {
   };
 
   struct node_stats {
-    node_stats(ssize_t l, ssize_t i)
+    node_stats(size_t l, size_t i)
         : leaf_nodes(l),
           internal_nodes(i) {
     }
@@ -897,8 +897,8 @@ class btree : public Params::key_compare {
       return *this;
     }
 
-    ssize_t leaf_nodes;
-    ssize_t internal_nodes;
+    size_t leaf_nodes;
+    size_t internal_nodes;
   };
 
  public: