Added a missing #include

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%404263

dlib/svm/svm_multiclass_linear_trainer.h

@@ -4,6 +4,7 @@
 #define DLIB_SVm_MULTICLASS_LINEAR_TRAINER_H__
 
 #include "svm_multiclass_linear_trainer_abstract.h"
+#include "structural_svm_problem.h"
 #include <vector>
 #include "../optimization/optimization_oca.h"
 #include "../matrix.h"