qtbase: remove gstreamer packageconfig

Nothing in qtbase uses gstreamer so the configure option was removed on the way
to 5.2 [1]. Fixes:

| -no-gstreamer: invalid command-line switch

[1] https://qt.gitorious.org/qt/qtbase/commit/e25a4faf4a33a758a51c1b96dfdead2f1675ccd7

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>