target/sparc/win_helper: silence the compiler warnings

target/sparc/win_helper: silence the compiler warnings

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
target/sparc/win_helper.c: In function ‘get_gregset’:
target/sparc/win_helper.c:304:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  304 |         trace_win_helper_gregset_error(pstate);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
target/sparc/win_helper.c:306:5: note: here
  306 |     case 0:
      |     ^~~~

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>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201211152426.350966-10-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>