add x86_64-pc-windows-gnu target to ci matrix

Author: ntsoftware

.github/workflows/windows.yml

@@ -16,6 +16,7 @@ jobs:
         os: [windows-latest]
         env:
         - TARGET: x86_64-pc-windows-msvc
+        - TARGET: x86_64-pc-windows-gnu
         - TARGET: i686-pc-windows-msvc
     runs-on: ${{ matrix.os }}
     steps: