lib/memcpy: fix warning on FreeBSD

lib/memcpy: fix warning on FreeBSD

Include struct timeval via sys/time.h.
http://pubs.opengroup.org/onlinepubs/009604499/basedefs/sys/time.h.html

--
In file included from lib/memcpy.c:7:
lib/../fio_time.h:14:45: warning: declaration of 'struct timeval' will not be visible outside of this function [-Wvisibility]
extern uint64_t mtime_since_tv(const struct timeval *, const struct timeval *);

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>