target/arm: Add missing break statement for Hypervisor Trap Exception

Reported by GCC9 when building with  -Wimplicit-fallthrough=2:

  target/arm/helper.c: In function ‘arm_cpu_do_interrupt_aarch32_hyp’:
  target/arm/helper.c:7958:14: error: this statement may fall through [-Werror=implicit-fallthrough=]
   7958 |         addr = 0x14;
        |         ~~~~~^~~~~~
  target/arm/helper.c:7959:5: note: here
   7959 |     default:
        |     ^~~~~~~
  cc1: all warnings being treated as errors

Fixes: b9bc21ff
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reported-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20190719111451.12406-1-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>