In the next patch I wish to add to mm.h prototypes of functions which take an
mmu_gather_t* argument.   To do this I must either:

a) include tlb.h in mm.h

   Not good - more nested includes when a simple forward decl is sufficient.

b) Add `typedef struct free_pte_ctx mmu_gather_t;' to mm.h.

   That's silly - it's supposed to be an opaque type.

   or

c) Remove the pesky typedef.

   Bingo.