patch 8.1.2413: cannot update ex_cmdidxs.h on MS-Windows

Problem:    Cannot update ex_cmdidxs.h on MS-Windows.
Solution:   Add build rules and dependencies. (Ken Takata, closes #5337)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 1a74d3b..65967b6 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -1411,6 +1411,13 @@
 	$(MAKE) /NOLOGO -f Makefile clean
 	cd ..
 
+# Run vim script to generate the Ex command lookup table.
+# This only needs to be run when a command name has been added or changed.
+# If this fails because you don't have Vim yet, first build and install Vim
+# without changes.
+cmdidxs: ex_cmds.h
+	vim --clean -X -u create_cmdidxs.vim
+
 test:
 	cd testdir
 	$(MAKE) /NOLOGO -f Make_dos.mak win32
@@ -1530,7 +1537,7 @@
 
 $(OUTDIR)/ex_cmds2.obj:	$(OUTDIR) ex_cmds2.c  $(INCL)
 
-$(OUTDIR)/ex_docmd.obj:	$(OUTDIR) ex_docmd.c  $(INCL)
+$(OUTDIR)/ex_docmd.obj:	$(OUTDIR) ex_docmd.c  $(INCL) ex_cmdidxs.h
 
 $(OUTDIR)/ex_eval.obj:	$(OUTDIR) ex_eval.c  $(INCL)