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 <firstname.lastname@example.org>