patch 8.1.2081: the spell.c file is too big

Problem:    The spell.c file is too big.
Solution:   Move the code for spell suggestions to a separate file. (Yegappan
            Lakshmanan, closes #4988)
diff --git a/src/proto.h b/src/proto.h
index ae4c63a..6d3b83d 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -202,6 +202,7 @@
 # include "sound.pro"
 # include "spell.pro"
 # include "spellfile.pro"
+# include "spellsuggest.pro"
 # include "syntax.pro"
 # include "tag.pro"
 # include "term.pro"