XML format: fix "Unrecognized parameter" warning

ApiMain::reportUnusedParams() is called before the XML formatter is
executed, so using "includexmlnamespace" or another such parameter
causes the incorrect generation of a warning. This patch corrects
the issue by explicitly excluding valid formatter parameters from
the list of unused parameters.

This does not need a release note because MediaWiki 1.20 did not
generate this kind of warning.

Change-Id: I269a6e07aa245099c811947d7832e1aa6fcfb9ec
Merge "XML format: fix "Unrecognized parameter" warning"