Add %%mingw_ninja_install macro
    
    In order to make the life of the mingw packages easier, let's add a
    %%mingw_ninja_install macro that will run `ninja -C install` in the
    proper directory (the one configured using %%mingw_meson macro), passing
    the correct DESTDIR to ninja.
    
    In case it's not done, po files are not correctly installed and
    %%mingw_find_lang macro will fail.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>