meson: include completion words header
currently gtksourcecompletionwords.h is not part of the public headers which is exported later on gtksource.h
Fixes
/usr/include/gtksourceview-5/gtksourceview/gtksource.h:65:10: fatal error: completion-providers/words/gtksourcecompletionwords.h: No such file or directory
65 | #include "completion-providers/words/gtksourcecompletionwords.h"
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~