lib: os: Fix unused variable set warning

Fixes following warning during compilation with llvm:

...
lib/os/mpsc_pbuf.c:428:8: warning: variable 'wrap' set but not used
  [-Wunused-but-set-variable]
        bool wrap;
             ^
1 warning generated.
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>