libertas: include sched.h on firmware.c

Do not assume we have our subsystem including this for us,
at least for older kernels this is not true. Lets just be
explicit about this requirement for the usage of wake_up().

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>