VeloClusterTracking - Fix warning from clang with -mavx2 enabled
Fixes a warning only visible when building with clang with avx2 enabled.
../Pr/PrPixel/src/VeloClusterTracking.cpp:78:17: warning: unused function 'set_used' [-Wunused-function]
inline void set_used( PlaneSoA* P, int h ) { ( (uint32_t*)P->used )[h / 32] |= 1 << ( h & 31 ); }