hw/intc/arm_gic_kvm: Fix build on aarch64

hw/intc/arm_gic_kvm: Fix build on aarch64

Remove unused debugging code to fix native building on aarch64. Without
this change, the following -Werr output inhibits make from completing.

  qemu/hw/intc/arm_gic_kvm.c:38:18: error: debug_gic_kvm defined but not used [-Werror=unused-const-variable=]
   static const int debug_gic_kvm = 0;
                    ^~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  qemu/rules.mak:60: recipe for target 'hw/intc/arm_gic_kvm.o' failed
  make[1]: *** [hw/intc/arm_gic_kvm.o] Error 1
  Makefile:205: recipe for target 'subdir-aarch64-softmmu' failed

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20161011163202.19720-1-cov@codeaurora.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>