mingw: define WIN32 explicitly

This helps VS Code's intellisense to figure out that we want to include
windows.h, and that we want to define the minimum target Windows version
as Windows Vista/2008R2.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -528,7 +528,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	COMPAT_OBJS += compat/mingw.o compat/winansi.o \
 		compat/win32/pthread.o compat/win32/syslog.o \
 		compat/win32/dirent.o
-	BASIC_CFLAGS += -DPROTECT_NTFS_DEFAULT=1
+	BASIC_CFLAGS += -DWIN32 -DPROTECT_NTFS_DEFAULT=1
 	EXTLIBS += -lws2_32
 	GITLIBS += git.res
 	PTHREAD_LIBS =