glib-mkenums: fix handling of forward enum declarations

glib-mkenums: fix handling of forward enum declarations

Given

    typedef enum MyFoo MyFoo;

glib-mkenums would get confused, not notice the ";", and then keep
skipping lines until it found one that started with a "{", possibly
even going into the next file.

Fix it to just ignore those lines instead (and also, to error out if
it hits eof while parsing an enum).

https://bugzilla.gnome.org/show_bug.cgi?id=669595