t0020-dirent.c: Add explicit returns to the callback functions

In particular, the one_entry() and dont_call_me() callback
functions require explicit returns, in order to suppress
some "control path" compiler warnings (from MS Visual C/C++).

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>

tests/t0020-dirent.c

@@ -105,6 +105,7 @@ static int one_entry(void *state, char *path)
 		}
 	}
 	test_die("unexpected path \"%s\"", path);
+	return -1;
 }
 
 
@@ -188,6 +189,7 @@ static walk_data empty = {
 static int dont_call_me(void *state, char *path)
 {
 	test_die("dont_call_me: unexpected callback!");
+	return -1;
 }
 
 BEGIN_TEST(empty)