Added generate-headers.sh for convenience purposes (#17413)

Author: kekekeks

native/Avalonia.Native/generate-headers.sh

@@ -0,0 +1,6 @@
+#/bin/bash
+set -e
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+cd $SCRIPT_DIR/../..
+dotnet run --project ./nukebuild/_build.csproj --target GenerateCppHeaders
+