Actually fix headless build, doh

headless/Headless.cpp

@@ -71,6 +71,7 @@ void NativeRender() { }
 
 std::string System_GetProperty(SystemProperty prop) { return ""; }
 void System_SendMessage(const char *command, const char *parameter) {}
+bool System_InputBoxGetWString(const wchar_t *title, const std::wstring &defaultvalue, std::wstring &outvalue) { return false; }
 
 #ifndef _WIN32
 InputState input_state;