Fix logic error double-importing system assemblies

Author: drewcassidy

KSPCommon.targets

@@ -9,7 +9,7 @@
   </ItemGroup>
 
   <!-- Prevent the compiler from acquiring or referencing any external reference assemblies -->
-  <PropertyGroup Condition=" '$(ReferenceKSPSystemAssemblies)' != 'true' ">
+  <PropertyGroup Condition=" '$(ReferenceKSPSystemAssemblies)' == 'true' ">
     <!-- Do not download the .NET Framework Target Pack NuGet package automatically -->
     <DisableTransitiveFrameworkReferenceDownloads>true</DisableTransitiveFrameworkReferenceDownloads>
     <!-- Instruct the compiler to not automatically reference mscorlib -->