patch 8.2.2408: MinGW: "--preprocessor" flag no longer supported

Problem:    MinGW: "--preprocessor" flag no longer supported.
Solution:   Remove the flag, use the defaults. (Christopher Wellons,
            closes #7741)
diff --git a/src/GvimExt/Make_ming.mak b/src/GvimExt/Make_ming.mak
index a6ab3ae..8d17142 100644
--- a/src/GvimExt/Make_ming.mak
+++ b/src/GvimExt/Make_ming.mak
@@ -49,8 +49,8 @@
 endif
 CXX := $(CROSS_COMPILE)g++
 WINDRES := $(CROSS_COMPILE)windres
-WINDRES_CXX = $(CXX)
-WINDRES_FLAGS = --preprocessor="$(WINDRES_CXX) -E -xc" -DRC_INVOKED
+# this used to have --preprocessor, but it's no longer supported
+WINDRES_FLAGS =
 LIBS :=  -luuid -lgdi32
 RES  := gvimext.res
 DEFFILE = gvimext_ming.def