util.cxx: Use abs() instead of labs()

util.cxx: Use abs() instead of labs()

Taking the absolute value of unsigned values is pointless, as reported
by clang:

util.cxx:1545:28: error: taking the absolute value of unsigned type 'unsigned long' has no effect [-Werror,-Wabsolute-value]
      unsigned min_score = labs(target.size() - it->size());
                           ^
util.cxx:1545:28: note: remove the call to 'labs' since unsigned values cannot be negative
      unsigned min_score = labs(target.size() - it->size());