Win32: Make timer functions backend independent

This is work towards allowing the Null backend to build for Windows
using the Win32 timer API, without the Win32 backend being enabled.

src/win32_time.h

@@ -0,0 +1,38 @@
+//========================================================================
+// GLFW 3.4 Win32 - www.glfw.org
+//------------------------------------------------------------------------
+// Copyright (c) 2002-2006 Marcus Geelnard
+// Copyright (c) 2006-2017 Camilla Löwy <elmindreda@glfw.org>
+//
+// This software is provided 'as-is', without any express or implied
+// warranty. In no event will the authors be held liable for any damages
+// arising from the use of this software.
+//
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it
+// freely, subject to the following restrictions:
+//
+// 1. The origin of this software must not be misrepresented; you must not
+//    claim that you wrote the original software. If you use this software
+//    in a product, an acknowledgment in the product documentation would
+//    be appreciated but is not required.
+//
+// 2. Altered source versions must be plainly marked as such, and must not
+//    be misrepresented as being the original software.
+//
+// 3. This notice may not be removed or altered from any source
+//    distribution.
+//
+//========================================================================
+
+#include <windows.h>
+
+#define _GLFW_PLATFORM_LIBRARY_TIMER_STATE  _GLFWtimerWin32   win32
+
+// Win32-specific global timer data
+//
+typedef struct _GLFWtimerWin32
+{
+    uint64_t            frequency;
+} _GLFWtimerWin32;
+