add missing header for std::unique_ptr in values.hpp

76f01fd4d3 removes the <memory> header
which is required to use std::unique_ptr for some builds
(e.g. archlinux using gcc 12.1.0).

include/exiv2/value.hpp

@@ -14,6 +14,7 @@
 #include <cstring>
 #include <iomanip>
 #include <map>
+#include <memory>
 
 // *****************************************************************************
 // namespace extensions