doc: use include_directories() to indicate both of source root and build root
doc: use include_directories() to indicate both of source root and build root

We can use include_directories() to indicate two paths from source root
and build root:
https://mesonbuild.com/Reference-manual.html#include_directories

Suggested-by: 4c163ab4e5b0b6191d958fad256acd257f4939c1@gnome

takaswie/libhinawa,takaswie/libhinawa