Fixed a bug in the get_filesystem_roots() roots function that

prevented it from compiling.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403179

dlib/dir_nav/dir_nav_kernel_2.h

@@ -650,7 +650,7 @@ namespace dlib
         typename queue_of_dir
         >
     typename enable_if<is_std_vector<queue_of_dir>,void>::type get_filesystem_roots (
-        const std::vector<directory>& roots
+        std::vector<directory>& roots
     )
     {
         roots.clear();