patch 8.2.2793: MS-Windows: string literals are writable with MSVC

Problem:    MS-Windows: string literals are writable with MSVC.
Solution:   Add the /GF compiler flag.  Make mch_write() safer. (Ken Takata,
            closes #8133)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 257f7e0..0f5611a 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -489,7 +489,7 @@
 #VIMRCLOC = somewhere
 #VIMRUNTIMEDIR = somewhere
 
-CFLAGS = -c /W3 /nologo $(CVARS) -I. -Iproto -DHAVE_PATHDEF -DWIN32 \
+CFLAGS = -c /W3 /GF /nologo $(CVARS) -I. -Iproto -DHAVE_PATHDEF -DWIN32 \
 		$(CSCOPE_DEFS) $(TERM_DEFS) $(SOUND_DEFS) $(NETBEANS_DEFS) $(CHANNEL_DEFS) \
 		$(NBDEBUG_DEFS) $(XPM_DEFS) \
 		$(DEFINES) -DWINVER=$(WINVER) -D_WIN32_WINNT=$(WINVER)