meson: Change the use of `enum_types` variable
meson: Change the use of `enum_types` variable

Although the `enum_types` variable stores source files from
generated enum types, the name does not reflect its real meaning,
storee source files. It now stores the name of the file with the
enum types description that allows avoiding typos.

A new variable called `enum_sources` is now used instead.

GNOME/libgit2-glib,GNOME/libgit2-glib