tools: compiler.h: add missing time.h

genimg_print_time uses time_t, but time.h is never included.
Linux gets away with this since types.h includes time.h.
Explicitly include the header file so building on e.g. FreeBSD
also works.

cc: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>