package-nuget-win.sh: enforce first parameter

This is POSIX and will print an error message with the usage if the
version parameter is not provided.

package-nuget-win.sh

 #! /bin/bash
 
-version=$1
+version=${1:?"Usage: $0 <libvlc version>"}
 downloadUrlx86="https://get.videolan.org/vlc/$version/win32/vlc-$version-win32.7z"
 downloadUrlx64="https://get.videolan.org/vlc/$version/win64/vlc-$version-win64.7z"