[TASK] Integrate system extension linkvalidator as submodule

Integrate system extension linkvalidator as Git submodule.
git://git.typo3.org/TYPO3v4/Extensions/linkvalidator.git

Change-Id: Idd931c0864692e4fc73d048192467205492f7497
Resolves: #32075
Reviewed-on: http://review.typo3.org/6911
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter