Fix index-clerk to build in all configurations
Fix index-clerk to build in all configurations

There appears to be a transitive dependency that 'bsl::size_t'
is assumed to be defined by one of the standard headers included
by this .cpp file, but at least on the cpp11/baseline, the only
use of 'size_t' appears to pull from what will becone 'native_std',
so explicitly #include the correct header directly from this cpp
file.

che2/bde,che2/bde,bloomberg/bde,che2/bde,bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,bloomberg/bde