Fix the following warning:
kernel/debug/gdbstub.c:1049:4: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
                           fallthrough;
                           ^
   include/linux/compiler_attributes.h:210:41: note: expanded from macro 'fallthrough'
   # define fallthrough                    __attribute__((__fallthrough__)
by placing the fallthrough; statement inside ifdeffery.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
                                gdb_cmd_detachkill(ks);
                                return DBG_PASS_EVENT;
                        }
-#endif
                        fallthrough;
+#endif
                case 'C': /* Exception passing */
                        tmp = gdb_cmd_exception_pass(ks);
                        if (tmp > 0)