Include necessary header files in radixtree.h.

Include necessary header files in radixtree.h.

When #include'ing radixtree.h with RT_SHMEM, it could happen to raise
compiler errors due to missing some declarations of types and
functions.

This commit also removes the inclusion of postgres.h since it's
against our usual convention.

Backpatch to v17, where radixtree.h was introduced.

Reviewed-by: Heikki Linnakangas, Álvaro Herrera
Discussion: https://postgr.es/m/CAD21AoCU9YH%2Bb9Rr8YRw7UjmB%3D1zh8GKQkWNiuN9mVhMvkyrRg%40mail.gmail.com
Backpatch-through: 17