Show build time

Show build time:

$ ./ninja --buildtime
ninja: no work to do.
build time: 00:00:00.000 (0 ms)

$ ./ninja --buildtime
[23/23] LINK ninja
build time: 00:00:00.113 (113 ms)
Aaah, I missed that (i used inttypes.h in C99 earlier) :/