patch 8.1.0673: functionality for signs is spread out over several files

Problem:    Functionality for signs is spread out over several files.
Solution:   Move most of the sign functionality into sign.c. (Yegappan
            Lakshmanan, closes #3751)
diff --git a/src/proto.h b/src/proto.h
index 92d9714..cbd4460 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -172,6 +172,9 @@
 #  include "sha256.pro"
 # endif
 # include "search.pro"
+# ifdef FEAT_SIGNS
+# include "sign.pro"
+# endif
 # include "spell.pro"
 # include "spellfile.pro"
 # include "syntax.pro"