Fix compilation error on Mac.
Fix compilation error on Mac.

_exit is declared in unistd.h

guanzidashu/guetzli_ios_metal,ianhuang-777/guetzli,dloeng/guetzli,szabadka/guetzli,google/guetzli,ianhuang-777/guetzli,guanzidashu/guetzli_ios_metal,google/guetzli,google/guetzli,dloeng/guetzli,dloeng/guetzli,guanzidashu/guetzli_ios_metal,szabadka/guetzli,ianhuang-777/guetzli,ianhuang-777/guetzli,google/guetzli,szabadka/guetzli,guanzidashu/guetzli_ios_metal