Unreviewed trivial buildfix after r147799.

Add an ifdef guard to ensure Qt uses its own monotonicallyIncreasingTime()
implemenation instead of the GLIB based one.

* wtf/CurrentTime.cpp:
(WTF):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147910 268f45cc-cd09-0410-ab3c-d52691b4dbfc