Remove argument's const qualifier from method declaration

Constness of a parameter is not part of a method's signature/ABI, even
though C++ for some reason allows it there.

src/decoders/TerminalCharacterDecoder.h

@@ -50,7 +50,7 @@ public:
      * @param count The number of characters
      * @param properties Additional properties which affect all characters in the line
      */
-    virtual void decodeLine(const Character *const characters, int count, LineProperty properties) = 0;
+    virtual void decodeLine(const Character *characters, int count, LineProperty properties) = 0;
 };
 
 }