ccan/base64: fix GCC warning.

ccan/base64: fix GCC warning.

```
ccan/ccan/base64/base64.c:146:71: error: argument 2 of type ‘char[3]’ with mismatched bound [-Werror=array-parameter=]
  146 | ssize_t base64_decode_tail_using_maps(const base64_maps_t *maps, char dest[3],
      |                                                                  ~~~~~^~~~~~~
In file included from ccan/ccan/base64/base64.c:2:
ccan/ccan/base64/base64.h:119:72: note: previously declared as ‘char *’
  119 | ssize_t base64_decode_tail_using_maps(const base64_maps_t *maps, char *dest,
      |                                                                  ~~~~~~^~~~
cc1: all warnings being treated as errors
make: *** [Makefile:817: ccan-base64.o] Error 1
```

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