[GTK] Clean up TemporaryLinkStubs.cpp
https://bugs.webkit.org/show_bug.cgi?id=112820

Reviewed by Philippe Normand.

Clean up the TemporaryLinkStubs.cpp file, removing unnecessary includes and prettifying the code.

No new tests - no new functionality.

* platform/gtk/TemporaryLinkStubs.cpp:
(WebCore):
(WebCore::getSupportedKeySizes):
(WebCore::signedPublicKeyAndChallengeString):
(WebCore::userIdleTime):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146379 268f45cc-cd09-0410-ab3c-d52691b4dbfc