wayland-util: do not export the wl_map_* API

Used only internally and explicitly marked as such with commit
cf04b0a1 ("Move private definitions and prototypes to new
zwayland-private.h")

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>