more pedantic silencing (#1763)

* prevent GCC from complaining about this unused parameter

dlib/dnn/layers.h

@@ -2241,8 +2241,7 @@ namespace dlib
             if (version != "add_prev_")
                 throw serialization_error("Unexpected version '"+version+"' found while deserializing dlib::add_prev_.");
         }
-
-        friend std::ostream& operator<<(std::ostream& out, const add_prev_& item)
+        friend std::ostream& operator<<(std::ostream& out, const add_prev_& )
         {
             out << "add_prev"<<id;
             return out;

dlib/image_loader/load_image.h

@@ -54,7 +54,7 @@ namespace dlib
 
             return UNKNOWN;
         }
-    };
+    }
 
 // ----------------------------------------------------------------------------------------
 

dlib/image_processing/scan_image_custom.h

@@ -105,7 +105,7 @@ namespace dlib
             double, 
             compute_object_score,
             ( const matrix<double,0,1>& w, const rectangle& obj) const
-        );
+        )
 
         template <typename fe_type>
         typename enable_if<has_compute_object_score<fe_type> >::type compute_all_rect_scores (

dlib/svm/sequence_labeler.h

@@ -64,7 +64,7 @@ namespace dlib
             bool, 
             template reject_labeling<matrix<unsigned long> >,
             (const typename T::sequence_type&, const matrix_exp<matrix<unsigned long> >&, unsigned long)const
-        );
+        )
 
         template <typename feature_extractor, typename EXP, typename sequence_type>
         typename enable_if<has_reject_labeling<feature_extractor>,bool>::type call_reject_labeling_if_exists (