Fix preprocessor use of _MSC_VER in include/winnt.h

[Gerald Pfeifer]

In all other places we check for the defined-ness of _MSC_VER before
using it; this addresses the remaining instance.

Gerald

ChangeLog:
Fix preprocessor use of _MSC_VER.

diff --git a/include/winnt.h b/include/winnt.h
index a77a607..222af22 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -296,7 +296,7 @@ extern "C" {
 #endif
 
 /* Eliminate Microsoft C/C++ compiler warning 4715 */
-#if (_MSC_VER > 1200)
+#if defined(_MSC_VER) && (_MSC_VER > 1200)
 # define DEFAULT_UNREACHABLE default: __assume(0)
 #else
 # define DEFAULT_UNREACHABLE