I'm sorry, but ew, no, gross! This is a terrible suggestion for several reasons:

1. It will only work for the last elements of the struct. If the struct is extended to have more elements later, elements that are added after the ifdef block will appear in the same memory location as those inside. Boom!

If you notice this and add to the middle of the struct, anything that depends on specific ordering of the data (packets, files?) will break. (Note: In some cases, extending these kinds of structs with more elements at the end of the struct is acceptable. However, see the paragraph above.)

2. You're not fooling anyone. Any code that uses this struct will need to include your header. They will be able to see that there is a conditional part of the struct, and will be able to define the appropriate macro to have everything included. (In my opinion, headers are a sort of implicit documentation. If you want to know what you can do with some bit of code, look at the header that is included.)

3. We already have an idiom for solving this sort of problem, called the opaque structure.

This sort of thing will likely cause more problems than it solves. Such a struct can't be moved, copied, or put in an array without potential memory corruption.

I'd suggest doing this:

  typedef struct {
    int a;
    int b;
    /* private */
    int c;
    int d;
  } STYPE;