xml: Shrink header size used to detect XML string embedded in URI

The previous string used to detect the XML header contained the
encoding hardcoded to "utf-8", which means that a XML string using the
"UTF-8" (or any other encoding really) wouldn't be recognized as a valid
XML string.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

xml.c

@@ -15,7 +15,7 @@
 #include <libxml/tree.h>
 #include <string.h>
 
-#define XML_HEADER "<?xml version=\"1.0\" encoding=\"utf-8\"?>"
+#define XML_HEADER "<?xml version=\"1.0\""
 
 static struct iio_context *
 xml_create_context(const struct iio_context_params *params,