Get access to the chpl_getNumCoresOnThisNode() declaration.

When I updated tasks-massivethreads recently I missed the fact that it
didn't already #include "chplsys.h", so it didn't have a declaration for
the chpl_getNumCoresOnThisNode() call I added.  Add the #include.
Merge pull request #278 from gbtitus/fix-massivethreads-getNumCores

Get access to the chpl_getNumCoresOnThisNode() declaration.

When I updated tasks-massivethreads recently I missed the fact that it
didn't already #include "chplsys.h", so it didn't have a declaration for
the chpl_getNumCoresOnThisNode() call I added. Add the #include.

Not reviewed.