Refactor name and type signature for xmlNop

Update xmlNop's name to xmlInputReadCallbackNop and its type signature
to match xmlInputReadCallback.

Fixes bug 786134.
diff --git a/libxml.h b/libxml.h
index 4fe56d2..2efa704 100644
--- a/libxml.h
+++ b/libxml.h
@@ -96,7 +96,7 @@
 #endif
 
 XMLPUBFUN xmlChar * XMLCALL xmlEscapeFormatString(xmlChar **msg);
-int xmlNop(void);
+int xmlInputReadCallbackNop(void *context, char *buffer, int len);
 
 #ifdef IN_LIBXML
 #ifdef __GNUC__