compiler warnings

[matthiaskrgr]

These warnings appeared today when using clang

[ 18%/13/1/72,90.988] Building CXX object src/CMakeFiles/wesnoth-game.dir/utils/irdya_datetime.cpp.o
In file included from ../src/utils/irdya_datetime.cpp:14:
../src/utils/irdya_datetime.hpp:60:25: warning: private field 'month' is not used [-Wunused-private-field]
        unsigned int year = 0, month = 0, day = 0;
                               ^
../src/utils/irdya_datetime.hpp:60:36: warning: private field 'day' is not used [-Wunused-private-field]
        unsigned int year = 0, month = 0, day = 0;
                                          ^
2 warnings generated.