Minor change to avoid compiler warning.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403869

dlib/svm/kernel_matrix.h

@@ -159,7 +159,7 @@ namespace dlib
 
         // Say we destructively alias if one of the vect* objects is actually item.
         bool alias_helper                   (const samp_type& item ) const { return are_same(item, vect1) || are_same(item, vect2); }
-        template <typename U> bool are_same (const samp_type& a, const U& b)         const { return false; }
+        template <typename U> bool are_same (const samp_type& , const U& )         const { return false; }
         bool are_same                       (const samp_type& a, const samp_type& b) const { return (&a == &b); }
     };