meson: arrangement for private headers
meson: arrangement for private headers

Some headers are just used for build, not for g-ir-scanner. For the reason,
sources_to_build variable is used but privates variable is more friendly.

This commit introduces privates variable.

Signed-off-by: Takashi Sakamoto <d5ec7d0cd8074757a1f5eb708bc105c3065a65ea@sakamocchi.jp>

takaswie/libhinawa,takaswie/libhinawa