patch 8.1.0724: build for MinGW fails

Problem:    Build for MinGW fails.
Solution:   Avoid specifying dependencies in included makefile.
diff --git a/src/testdir/Make_dos.mak b/src/testdir/Make_dos.mak
index 2623fb1..5776a79 100644
--- a/src/testdir/Make_dos.mak
+++ b/src/testdir/Make_dos.mak
@@ -132,5 +132,7 @@
 	$(VIMPROG) -u gui_preinit.vim -U gui_init.vim $(NO_PLUGINS) -S runtest.vim $*.vim
 	@del vimcmd
 
+test_options.res test_alot.res: opt_test.vim
+
 opt_test.vim: ../option.c gen_opt_test.vim
 	$(VIMPROG) -u NONE -S gen_opt_test.vim --noplugin --not-a-term ../option.c