Made static SiPixelRecHistQuality::thePacking const

The static analyzer spotted this as a potential problem. All the public member data for the class are initialized in the constructor and are not meant to change. Changing all member functions to const and then making the static a const appears to compile just fine.
Merge pull request #1774 from Dr15Jones/make_SiPixelRecHitQuality_thePacking_const

Multithreading fixes -- Made static SiPixelRecHistQuality::thePacking const