KSPCommon.props: add System.Core.dll to references

Author: JonnyOThan

KSPCommon.props

@@ -79,6 +79,10 @@
       <Name>System (KSP/Mono)</Name>
       <Private>False</Private>
     </Reference>
+    <Reference Include="$(ManagedPath)\System.Core.dll">
+      <Name>System (KSP/Mono)</Name>
+      <Private>False</Private>
+    </Reference>
     <Reference Include="$(ManagedPath)\mscorlib.dll">
       <Name>System.Core (KSP/Mono)</Name>
       <Private>False</Private>