rational: make AVRational conversion const

Change-Id: Ie9f1c3c23ac9e062e85cfce57aee3b1f9a6cea38

src/rational.h

@@ -45,7 +45,7 @@ public:
 
     // Define conversions to and from AVRational (equivalent)
     rational(AVRational r) : num_(r.num), den_(r.den) {};
-    operator AVRational() { return AVRational{num_, den_}; }
+    operator AVRational() const { return AVRational{num_, den_}; }
 
     // Normal copy constructors and assignment operators