crush/CrushWrapper.cc: don't pass c_str() result to std::string argument

crush/CrushWrapper.cc: don't pass c_str() result to std::string argument

Fix issue found by cppcheck:

[src/crush/CrushWrapper.cc:352]: (performance) Passing the result of
  c_str() to a function that takes std::string as argument no. 4 is
  slow and redundant.
[src/crush/CrushWrapper.cc:388]: (performance) Passing the result of
  c_str() to a function that takes std::string as argument no. 4 is
  slow and redundant.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>