Other gstreamer repositories have their own valgrind suppression file
directly in the repository.
Add a suppression file to the core gstreamer repository too, this makes
it easier to use it with gst-build which does not check out the common
module.
This is also a little step towards the removal of the common submodule.
NOTE: the added file is the latest version from the "common" repository
but it has been renamed from gst.supp to gstreamer.supp for symmetry
with the suppression files in the other repositories.
