ccan/rune: compile without warnings on -O3.

ccan/rune: compile without warnings on -O3.

```
ccan/ccan/rune/rune.c: In function ‘rune_alt_single_int’:
ccan/ccan/rune/rune.c:257:5: error: ‘runeval_int’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  257 |  if (cond)
      |     ^
ccan/ccan/rune/rune.c:305:6: note: ‘runeval_int’ was declared here
  305 |  s64 runeval_int;
      |      ^~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [Makefile:919: ccan-rune-rune.o] Error 1
```

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