Fix PrepSDKCall_SetSignature on Windows

[Rostu13]

Server will be crash with next code on windows

@asherkin answered this
https://canary.discordapp.com/channels/335290997317697536/335290997317697536/746316421134548993

#include <sdktools>

public void OnPluginStart()
{
    StartPrepSDKCall(SDKCall_Static);
    PrepSDKCall_SetSignature(SDKLibrary_Engine, "@CreateInterface", 0);
    PrepSDKCall_AddParameter(SDKType_String, SDKPass_Pointer);
    PrepSDKCall_AddParameter(SDKType_PlainOldData, SDKPass_Pointer, VDECODE_FLAG_ALLOWNULL);
    PrepSDKCall_SetReturnInfo(SDKType_PlainOldData, SDKPass_Plain);

    Handle h= EndPrepSDKCall();

    int IVModelRender = SDKCall(h, "VEngineModel016", 0);

    PrintToServer("IVModelRender 0x%X", IVModelRender);

    delete h;
}

This PR fixed it