Add the required SDL_win32_main file for mingw32 builds.

src/conio/GNUmakefile

@@ -5,6 +5,7 @@ include $(SRC_ROOT)/build/Common.gmake	# defines clean and output directory rule
 CFLAGS += $(XPDEV-MT_CFLAGS) $(CIOLIB-MT_CFLAGS)
 
 ifeq ($(os),win32)
+ OBJS	+=	$(MTOBJODIR)$(DIRSEP)SDL_win32_main$(OFILE)
  OBJS	+=	$(MTOBJODIR)$(DIRSEP)win32cio$(OFILE)
 else
  OBJS	+=	$(MTOBJODIR)$(DIRSEP)curs_cio$(OFILE)