travis: updated mingw cross compilation test
travis: updated mingw cross compilation test

- only compiles zstd CLI
- enforces zero warning tolerance policy

note : mingw compilation of the library complains (warning) about -fPIC flag.
It mostly ignores it, so it's not a big issue.
Still, it would need to be fixed if library cross-compilation becomes part of CI.
At this stage, it's unclear if this is a scenario.

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd