Add const to constexpr non-static member function in SiPixelCluster.h

In C++14 rule 'constexpr implies const' for non-static member functions is no more.
Explicitly add 'const' modifier to keep behavior identical between C++11 and C++14.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1520 from davidlt/resolve-cpp14-warnings

C++14 fixes -- Add const to constexpr non-static member function in SiPixelCluster.h