Ensure that qtandroidextras is checked out when building qtconnectivity

This is required for the upcoming Android port.

Only the Android backend will really need it but the CI doesn't know
about conditional git checkouts. All other platforms will simply not
build/use it.

Change-Id: I32dcc2e616dcda05aca1957512d03e6308dbf45f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>