ARM: tegra: cpuidle: Add missing cpuidle.h include

Some of the functions implemented are flagged as not having a prototype
defined when building with W=1. Include the header to avoid these build
warnings and add a prototype as well as a dummy implementation for the
tegra_cpuidle_pcie_irqs_in_use() function.

Signed-off-by: Thierry Reding <treding@nvidia.com>
diff --git a/arch/arm/mach-tegra/cpuidle-tegra20.c b/arch/arm/mach-tegra/cpuidle-tegra20.c
index 7469347..afcee04 100644
--- a/arch/arm/mach-tegra/cpuidle-tegra20.c
+++ b/arch/arm/mach-tegra/cpuidle-tegra20.c
@@ -30,6 +30,7 @@
 #include <asm/smp_plat.h>
 #include <asm/suspend.h>
 
+#include "cpuidle.h"
 #include "flowctrl.h"
 #include "iomap.h"
 #include "irq.h"