ios: adding dsymutil build step

The lldb debugger sometime returns misleading debug information
unless a dsym has been created.
This creates a build step to do it.

Task-number: QTCREATORBUG-11580
Change-Id: I7acf2e539cf189d0237e1d502fab2837f17aa489
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>