amount: make it work with gcc-4.8.
```
In file included from bitcoin/chainparams.h:7:0,from bitcoin/chainparams.c:1:
./common/amount.h:36:11: error: initializer element is not constant
((struct amount_sat){(constant) + AMOUNT_MUST_BE_CONST(constant)})
^
bitcoin/chainparams.c:20:21: note: in expansion of macro ‘AMOUNT_SAT’
.max_funding = AMOUNT_SAT((1 << 24) - 1),
^
./common/amount.h:36:11: error: (near initialization for ‘networks[0].max_funding’)
((struct amount_sat){(constant) + AMOUNT_MUST_BE_CONST(constant)})
^
bitcoin/chainparams.c:20:21: note: in expansion of macro ‘AMOUNT_SAT’
.max_funding = AMOUNT_SAT((1 << 24) - 1),
```
Fixes: #2404
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
02faadfb93