Rename template parameter: "I" -> "II"

C99 defines the "I" macro in complex.h. qobjectdefs_impl.h can be
indirectly included in user code, which raises the possibility of a name
clash if the user's compiler supports C99 and the user includes
complex.h

Change-Id: Ie79ec7baf2d49a34b66a01556c7e57324303dc04
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>