bfd: add missing include <time.h>

bdfio.c is defining bfd_get_current_time which is returning a time_t.
This type is defined in time.h and thus, must be included in bfd main
header to avoid undefined type when include bfd.h.

Note that most of the time, <time.h> is pulled by <sys/stat.h> already
included in bfd.h. That's why it went unnoticed.