Include <unistd.h>, as we are otherwise missing definitions on some systems.
Include <unistd.h>, as we are otherwise missing definitions on some systems.

BUG=http://code.google.com/p/seccompsandbox/issues/detail?id=19
TEST=make test
Review URL: https://chromiumcodereview.appspot.com/10384174
cnbird1999/seccompsandbox,cnbird1999/seccompsandbox,ericmckean/seccompsandbox,ericmckean/seccompsandbox,cnbird1999/seccompsandbox,ericmckean/seccompsandbox