parse: make suggestions for unknown options

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/parse.h b/parse.h
index a9d726d..15f2e06 100644
--- a/parse.h
+++ b/parse.h
@@ -95,6 +95,8 @@
 extern int check_str_time(const char *p, long long *val, int);
 extern int str_to_float(const char *str, double *val, int is_time);
 
+extern int string_distance(const char *s1, const char *s2);
+
 /*
  * Handlers for the options
  */