Fix build script for Windows client.

Hi, I'm Chen Gong <chengong@google.com> from IME team, working on the client input method framework.

Commit https://github.com/lotem/google-input-tools/commit/1d5a10197c180587e7b851125982eb3bdd7edad2 fixed a bug in a script modifying all.sln file for mixed-platform (x86 & x64) builds.
Without this fix, the solution works well in the MSVS IDE, but fails to do a release build with msbuild.exe on the console.