iOS: silence compiler warning about unimplemented 'selectionRectsForRange'

Implement a dummy method to silence the compiler.
After testing, this method seems to never be called. Which is
good, since the current IM API in Qt have little to offer to resolve
what is being asked. Until a need arise, we just return
an empty array.

Change-Id: I573eb8205a7e635a46d487ae175fb46e3a602001
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>