target/ppc/excp_helper: Add a fallthrough for fix compiler warning

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
../target/ppc/excp_helper.c: In function ‘powerpc_excp’:
../target/ppc/excp_helper.c:529:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
  529 |         msr |= env->error_code;
      |         ~~~~^~~~~~~~~~~~~~~~~~
../target/ppc/excp_helper.c:530:5: note: here
  530 |     case POWERPC_EXCP_HDECR:     /* Hypervisor decrementer exception         */
      |     ^~~~

Add the corresponding "fall through" comment to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Message-Id: <20201028055107.2170401-1-kuhn.chenqun@huawei.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>