Make static a const in SiStripFecCabling::fecs()

Silences Clang Static Analyzer warning.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1532 from davidlt/clean-up-static-SiStripFecCabling.h

Multithreading fixes -- Make static a const in SiStripFecCabling::fecs()