Small fix in pg_rewind (redundant declaration)

Hello hackers.
I've stumbled upon a strange code.
In src/bin/pg_rewind/datapagemap.h we decalre:
extern void datapagemap_destroy(datapagemap_t *map);
But nowhere is implemented. I think the declaration of this function must be
removed.
I'm not sure that this trivial things needed patch.

Thanks.

--
YUriy Zhuravlev
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company