util: Add __declspec(noreturn) to _debug_assert_fail().

Mostly for consistency; as MSVC's static source code analysis doesn't
seem to rely on assertions, but instead on different kind of source
annotations( http://msdn.microsoft.com/en-us/library/hh916383.aspx ).

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/util/u_debug.h

@@ -151,6 +151,9 @@ void debug_print_format(const char *msg, unsigned fmt );
 long
 debug_get_num_option(const char *name, long dfault);
 
+#ifdef _MSC_VER
+__declspec(noreturn)
+#endif
 void _debug_assert_fail(const char *expr, 
                         const char *file, 
                         unsigned line,