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>