All users are supposed to include files from nm-utils by fully specifying
the path. -I.*shared/nm-utils is wrong.

Only, systemd code likes to include "siphash24.h" directly. Instead of
adding "-Ishared/nm-utils" to the search path, add an intermediary
header to sd-adapt. Note, that in the meantime we anyway should rework
siphash24 to use shared/c-siphash instead.

This also fixes build for meson, which was broken recently.