The operator< should be const, so it can be applied to const tree_struct.

This resolves FTBFS on FreeBSD.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit 2e4f150a47)

src/qalculate_tde_utils.h

@@ -49,7 +49,8 @@ struct tree_struct {
 			it->sort();
 		}
 	}
-	bool operator < (tree_struct &s1) const {
+	bool operator < (const tree_struct &s1) const
+	{
 		return item < s1.item;
 	}
 };