Update makefile to build fiptool for Windows

Although support for building fiptool on a Windows host was present,
the binary was not built when the top level makefile was invoked.

This patch makes the necessary changes to the to support building of
fiptool on a Windows host PC from the main makefile.

Change-Id: I0c01ba237fa3010a027a1b324201131210cf4d7c
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
diff --git a/make_helpers/windows.mk b/make_helpers/windows.mk
index 5ab8bdc..26ea88e 100644
--- a/make_helpers/windows.mk
+++ b/make_helpers/windows.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016-2018, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2016-2020, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -86,3 +86,5 @@
 		$$(CC) $$(TF_CFLAGS) $$(CFLAGS) -x c -c - -o $1
 endef
 
+MSVC_NMAKE := nmake.exe
+