[PicturePlayer] Fix an accidental rename of a function

Introduced here: f4606e9#diff-6cc74809d9a7c6d6dd81891b6c810e548561a48681e43480af6ca402c3da85cbL571

lib/python/Plugins/Extensions/PicturePlayer/ui.py

@@ -578,7 +578,7 @@ def start_decode(self):
 		self.picload.startDecode(self.filelist[self.index])
 		self["point"].show()
 
-	def __next__(self):
+	def next(self):
 		self.index += 1
 		if self.index > self.maxentry:
 			self.index = 0