Added missing requires clause.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402639

dlib/threads/thread_pool_extension_abstract.h

@@ -24,6 +24,8 @@ namespace dlib
             unsigned long num_threads
         );
         /*!
+            requires
+                - num_threads > 0
             ensures
                 - num_threads_in_pool() == num_threads
             throws