ARM: OMAP: Fix warning in mach-omap2

Fix warning

arch/arm/mach-omap2/timer-gp.c: In function
'omap2_gp_timer_init':
arch/arm/mach-omap2/timer-gp.c:70: warning: implicit
declaration of function 'setup_irq'

Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>