Fix cmake warning

CMake Warning (dev) in CMakeLists.txt:
  Syntax Warning in cmake code at

    /home/mparent/src/externe/github.com/rozofs/build-area/rozofs-1.3.5/CMakeLists.txt:168:31

  Argument not separated from preceding token by whitespace.
This warning is for project developers.  Use -Wno-dev to suppress it.