Convert pthread_join() return code to long not size_t.
Convert pthread_join() return code to long not size_t.

- The compiler complains about casting from void* to an int.
- A size_t is the same size as a void*, but it is unsigned.
- A long is signed, and should be the same size on all supported platforms.

Change-Id: Icce51e0dfb9b779016494699f89ed7c7e2f2eb36
Signed-off-by: Christopher M. Cantalupo <9cf3ecca60a3dd7112ab0e7250bedb8a752e3958@intel.com>

sssylvester/geopm,cmcantalupo/geopm,sssylvester/geopm,geopm/geopm,sssylvester/geopm,cmcantalupo/geopm,geopm/geopm,geopm/geopm,geopm/geopm,cmcantalupo/geopm,cmcantalupo/geopm,sssylvester/geopm,cmcantalupo/geopm,geopm/geopm