meson: code refactoring to remove useless assignments for include headers

meson: code refactoring to remove useless assignments for include headers

In meson build, gnome.mkenums_simple() and gnome.genmarshal()
voluntarily install headers, therefore it's useless to install
them explicitly by install_headers().

This commit removes useless assignments for include headers. I note that
gnome.generate_gir() fails to parse codes when enums are prior to
headers and sources. This commit also reorder entries for safe.

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