In 0.3.4 and later, we declare write_array as:
extern struct bw_array_t *write_array;
...
typedef struct bw_array_t bw_array_t;

But in 0.2.9, we declare write_array as:
typedef struct bw_array_t bw_array_t;
extern bw_array_t *write_array;

And then again in rephist.c:
typedef struct bw_array_t bw_array_t;

So some compilers fail with a duplicate declaration error.

We backport 684b396c, which removes the duplicate declaration.
And this commit deals with the undeclared type error.

Backports a single line from merge commit 813019cc.

Fixes bug 30184; not in any released version of Tor.