CI: set -Wno-error=maybe-uninitialized
After the update of msys2-w32api from v11.0.1 to current master (and soon to be v12)
we get: winsup/cygwin/exceptions.cc:1736:33: error: '<anonymous>' may be used uninitialized [-Werror=maybe-uninitialized]
Ignore it like the rest.
Fixes #214
https://github.com/msys2/msys2-runtime/commit/3f5eb36d40c9bb4e5ffe977b4ba21b4f4f9bf671