[test][sanitizer] Add fork include to the test

Author: vitalybuka

compiler-rt/test/sanitizer_common/TestCases/Posix/fork_threaded.cpp

@@ -16,6 +16,7 @@
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <unistd.h>
 
 #include "sanitizer_common/sanitizer_specific.h"