bsd-user: Silence warnings about missing fallthrough statement

When compiling with -Werror=implicit-fallthrough, the compiler complains
about a missing fallthrough annotation in this file. Looking at the code,
the fallthrough is indeed wanted here, so let's add a proper comment.

Message-Id: <20201217154138.1547274-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

bsd-user/main.c

@@ -512,6 +512,7 @@ void cpu_loop(CPUSPARCState *env)
         case 0x141:
             if (bsd_type != target_freebsd)
                 goto badtrap;
+            /* fallthrough */
         case 0x100:
 #endif
             syscall_nr = env->gregs[1];