Fix build breakage

The MinikinFont class was missing a destructor. The build error was
not caught because incremental builds didn't see fit to relink after
I deleted one of the source files (that contained the impl of this
destructor).

Change-Id: Ic72d56fe28316cd2b2f808910e34ca6f177a1220

engine/src/flutter/include/minikin/MinikinFont.h

@@ -38,8 +38,7 @@ public:
     void Ref() { mRefcount_++; }
     void Unref() { if (--mRefcount_ == 0) { delete this; } }
 
-    //MinikinFont();
-    virtual ~MinikinFont() = 0;
+    virtual ~MinikinFont() { };
 
     virtual bool GetGlyph(uint32_t codepoint, uint32_t *glyph) const = 0;