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_cyg_ming.mak b/src/Make_cyg_ming.mak
index 52156dd..11ce0fc 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -746,6 +746,7 @@
 	$(OUTDIR)/screen.o \
 	$(OUTDIR)/search.o \
 	$(OUTDIR)/sha256.o \
+	$(OUTDIR)/sign.o \
 	$(OUTDIR)/spell.o \
 	$(OUTDIR)/spellfile.o \
 	$(OUTDIR)/syntax.o \