[BHaType]

Try this gamedata

PHP Code:

"Games"
{
    "left4dead2"
    {
        "Functions"
        {
            "PassEntityFilter"
            {
                "signature"        "PassEntityFilter"
                "callconv"        "cdecl"
                "return"        "bool"
                "this"            "ignore"
                "arguments"
                {
                    "a1"
                    {
                        "type"    "cbaseentity"
                    }
                    "a2"
                    {
                        "type"    "cbaseentity"
                    }
                }
            }
        }
        "Signatures"
        {
            "PassEntityFilter"
            {
                "library" "server"
                "windows"        "\x55\x8B\xEC\x57\x8B\x7D\x0C\x85\xFF\x75"
            }
        }
    }
} 


If this doesn't help then change the

"callconv"        "cdecl"

to this

"callconv"        "thiscall"