A minor change to avoid a compiler warning from gcc.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403997

dlib/svm/svm_nu_trainer.h

@@ -196,7 +196,7 @@ namespace dlib
             alpha = pointwise_multiply(alpha,y)/rho;
 
             // count the number of support vectors
-            const long sv_count = sum(alpha != 0);
+            const long sv_count = (long)sum(alpha != 0);
 
             scalar_vector_type sv_alpha;
             sample_vector_type support_vectors;