Fixes #110

The original fix in commit 64ed308 never worked because PREMode
defaulted to INDCPA, not NOT_SET. This caused GetParamsPK() to
always return Q×P basis for HYBRID, even when PRE is not needed.

Changes:
- Set default PREMode to NOT_SET in gen-cryptocontext-params-defaults.h
- Update pre-buffer.cpp to explicitly set PREMode=INDCPA

Co-authored-by: Burak Kucutktopal <burak.kucuktopal@nxp.com>