Add OS condition to project file for Windows NT

Updated the `<Project>` element in the project file to include a `Condition` attribute that checks for the Windows NT operating system. This change enhances compatibility by ensuring the project file is only applicable when running on Windows NT, reducing potential issues on other operating systems.

Author: Capnode

Retroworks.RCBus.Windows/Retroworks.RCBus.Windows.wapproj

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" Condition="'$(OS)' == 'Windows_NT'">
   <PropertyGroup Condition="'$(VisualStudioVersion)' == '' or '$(VisualStudioVersion)' &lt; '15.0'">
     <VisualStudioVersion>15.0</VisualStudioVersion>
   </PropertyGroup>