fix: explicitly specify Blocklist::size() return type as size_t

Fixes building with C++20/C++23
error: no matching function for call to 'size'
function 'size' with deduced return type cannot be used before it is defined

Signed-off-by: Dzmitry Neviadomski <nevack.d@gmail.com>

libtransmission/blocklist.h

@@ -78,7 +78,7 @@ private:
 
         [[nodiscard]] bool contains(tr_address const& addr) const;
 
-        [[nodiscard]] auto size() const
+        [[nodiscard]] size_t size() const
         {
             ensureLoaded();