Fix errors thrown due to platform libxml2 version upgrade from 2.11.5 -> 2.12.5

Fix errors thrown due to platform libxml2 version upgrade from 2.11.5 -> 2.12.5

libxml 2.10.5 :
  typedef void (*xmlStructuredErrorFunc) (void *userData, xmlError *error);

libxml 2.12.5 :
  typedef void (*xmlStructuredErrorFunc) (void *userData, const xmlError *error);

Due to update in above func ptr signature, faced below error on blink side.

Error:
  [ 9063s] ../third_party/blink/renderer/core/xml/parser/xml_document_parser_scope.h:41:3:
    note: candidate constructor not viable: no known conversion from 'void (void *, xmlError *)' (aka 'void (void *, _xmlError *)')
      to 'xmlStructuredErrorFunc' (aka 'void (*)(void *, const _xmlError *)') for 3rd argument

Commit [1] enabled third_party/libxml, as QB build was throwing above error.
But due to unexpected crash on TERMS webview app, [1] is now partially reverted to
use system libxml2, and additional changes are done on Blink side to handle above error.

[1] 98565b6bd0df2aaa35f21f30f0bd81ed718b2cf5

Change-Id: I963a42fb3fd1f16d41aca5d85e38f18b626a9cbc
Signed-off-by: Gajendra N <gajendra.n@samsung.com>