fix CharacterWidthDeterminer to be an abstract class

src/core/CharacterWidthDeterminer.h

@@ -6,7 +6,7 @@
 class CharacterWidthDeterminer
 {
 public:
-    virtual int operator()(uint32_t character);
+    virtual int operator()(uint32_t character) = 0;
 };
 
 #endif