talloc: remove const warning in _info example.

Compiler:
/home/rusty/devel/cvs/ccan/ccan/talloc/_info: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/talloc/_info:84:9: warning: passing argument 2 of ‘open_output_cmd’ discards ‘const’ qualifier from pointer target type [enabled by default]
/home/rusty/devel/cvs/ccan/ccan/talloc/_info:52:24: note: expected ‘char *’ but argument is of type ‘const char *’

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/talloc/_info

@@ -49,7 +49,8 @@
  *	}
  *
  *	// This function opens a writable pipe to the given command.
- *	static struct command *open_output_cmd(const void *ctx, char *fmt, ...)
+ *	static struct command *open_output_cmd(const void *ctx,
+ *					       const char *fmt, ...)
  *	{
  *		va_list ap;
  *		struct command *cmd = talloc(ctx, struct command);

ccan/tcon/test/compile_fail-tcon_cast.c

@@ -25,5 +25,5 @@ int main(int argc, char *argv[])
 
 	icon.raw.p = NULL;
 	x = tcon_cast(&icon, tc1, icon.raw.p);
-	return 0;
+	return x != NULL ? 0 : 1;
 }