intc/arm_gic: Fix gic_irq_signaling_enabled() for vCPUs

Correct the indexing into s->cpu_ctlr for vCPUs.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Message-id: 20201214222154.3480243-2-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>