patch 8.1.1230: a lot of code is shared between vim.exe and gvim.exe

Problem:    A lot of code is shared between vim.exe and gvim.exe.
Solution:   Optionally put the shared code in vim.dll. (Ken Takata,
            closes #4287)
diff --git a/Filelist b/Filelist
index a6cc76c..7189278 100644
--- a/Filelist
+++ b/Filelist
@@ -454,6 +454,7 @@
 		src/iscygpty.h \
 		src/iid_ole.c \
 		src/os_dos.h \
+		src/os_w32dll.c \
 		src/os_w32exe.c \
 		src/os_win32.c \
 		src/os_mswin.c \