Add type cast to foreach_internal's loop variable.

Add type cast to foreach_internal's loop variable.

C++ requires explicitly casting void pointers to the appropriate
pointer type, which means the foreach_ptr macro cannot be used in
C++ code without this change.

Author: Jelte Fennema-Nio
Reviewed-by: Bruce Momjian
Discussion: https://postgr.es/m/CAGECzQSYG3QfHrc-rOk2KbnB9iJOd7Qu-Xii1s-GTA%3D3JFt49Q%40mail.gmail.com
Backpatch-through: 17