linux: turn on -Wextra

sandbox/linux/suid/process_util_linux.c

Index: sandbox/linux/suid/process_util_linux.c
diff --git a/sandbox/linux/suid/process_util_linux.c b/sandbox/linux/suid/process_util_linux.c
index 9f40f39be19876a40736cf4dc0ab04079f4e7aac..7b312747433ab16341c44816b6cdba964b6f9b03 100644
--- a/sandbox/linux/suid/process_util_linux.c
+++ b/sandbox/linux/suid/process_util_linux.c
@@ -38,7 +38,7 @@ bool AdjustOOMScore(pid_t process, int score) {
 
   char buf[3];  // 0 <= |score| <= 15;
   snprintf(buf, sizeof(buf), "%d", score);
-  size_t len = strlen(buf);
+  ssize_t len = strlen(buf);
 
   ssize_t bytes_written = write(fd, buf, len);
   close(fd);