ci: add MinGW cross-compilation in GHA

ci: add MinGW cross-compilation in GHA

Add mingw cross compilation in our public CI so that users with their
own github repository have a first level of checks for Windows compilation
before submitting to the mailing list.
This does not replace our better checks in other entities of the CI.

Only the helloworld example is compiled (same as what is tested in
test-meson-builds.sh).

Note: the mingw cross compilation toolchain (version 5.0) in Ubuntu
18.04 was broken (missing a ENOMSG definition).

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>