[GL] Add missing const to SegmentIterator::op==().

gcc-8.3 complains otherwise about the comparison
operator being missing for two constant SegmentIterators.

GeoLib/Polyline.cpp

@@ -524,12 +524,12 @@ LineSegment Polyline::SegmentIterator::operator*()
     return _polyline->getSegment(_segment_number);
 }
 
-bool Polyline::SegmentIterator::operator==(SegmentIterator const& other)
+bool Polyline::SegmentIterator::operator==(SegmentIterator const& other) const
 {
     return !(*this != other);
 }
 
-bool Polyline::SegmentIterator::operator!=(SegmentIterator const& other)
+bool Polyline::SegmentIterator::operator!=(SegmentIterator const& other) const
 {
     return other._segment_number != _segment_number ||
            other._polyline != _polyline;

GeoLib/Polyline.h

@@ -72,9 +72,9 @@ public:
 
         LineSegment operator*();
 
-        bool operator==(SegmentIterator const& other);
+        bool operator==(SegmentIterator const& other) const;
 
-        bool operator!=(SegmentIterator const& other);
+        bool operator!=(SegmentIterator const& other) const;
 
         SegmentIterator& operator+=(std::vector<GeoLib::Point>::difference_type n);