OMAP2: PM: Kill compile warnings at arch/arm/mach-omap2/pm.c

Get rid of the following warnings:
arch/arm/mach-omap2/pm.c: In function 'omap2_enter_full_retention':
arch/arm/mach-omap2/pm.c:509: warning: implicit declaration of function 'omap2_gpio_prepare_for_retention'
arch/arm/mach-omap2/pm.c:536: warning: implicit declaration of function 'omap2_gpio_resume_after_retention'
Signed-off-by: Felipe Balbi <felipe.lima@indt.org.br>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/pm.c

@@ -39,6 +39,7 @@
 #include <asm/arch/irqs.h>
 #include <asm/arch/clock.h>
 #include <asm/arch/sram.h>
+#include <asm/arch/gpio.h>
 #include <asm/arch/pm.h>
 #include <asm/arch/mux.h>
 #include <asm/arch/dma.h>