couple of fixes to get IECoreNuke to compile

The two errors addressed were:
- error: ‘replace_knobs’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
note: declarations in dependent base ‘DD::Image::Op’ are not found by unqualified lookup
note: use ‘this->replace_knobs’ instead
- error: variable ‘presetIndex’ set but not used [-Werror=unused-but-set-variable]