Documentation/timers/hpet_example: drop duplicate header files

Documentation/timers/hpet_example.c: fcntl.h is included more than once.
Documentation/timers/hpet_example.c: signal.h is included more than once.

Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>