Fix Log2Histogram->Add() header
Fix Log2Histogram->Add() header

The parameter sent to the histogram `Add()` function
is usualy the difference between two uint64_t values
(returned by `platform_monotonic_ns()`). Currently, the
header declares a 32 bits integer parameter. This may cause
an overflow when callbacks take longer than 2^32-1 ns and
can generate eronate results.
Fix the function header.

cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs