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/Make_ivc.mak b/src/Make_ivc.mak
index c8c1c2f..784cab9 100644
--- a/src/Make_ivc.mak
+++ b/src/Make_ivc.mak
@@ -257,6 +257,7 @@
 	"$(INTDIR)/screen.obj" \
 	"$(INTDIR)/search.obj" \
 	"$(INTDIR)/sha256.obj" \
+	"$(INTDIR)/sign.obj" \
 	"$(INTDIR)/spell.obj" \
 	"$(INTDIR)/spellfile.obj" \
 	"$(INTDIR)/syntax.obj" \
@@ -675,6 +676,10 @@
 # End Source File
 # Begin Source File
 
+SOURCE=.\sign.c
+# End Source File
+# Begin Source File
+
 SOURCE=.\spell.c
 # End Source File
 # Begin Source File