Add missing msvc-stlport unusable state for the interval library

[SVN r22357]

status/explicit-failures-markup.xml

@@ -369,6 +369,7 @@
             <toolset name="borland"/>
             <toolset name="borland-5.6.4"/>
             <toolset name="msvc"/>
+            <toolset name="msvc-stlport"/>
             <toolset name="vc7"/>
             <toolset name="intel-win32-7.1-vc6-stlport-4.5.3"/>
         </mark-unusable>