kernel: debug: Fix unreachable code in gdb_serial_stub()

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>

kernel/debug/gdbstub.c

@@ -1045,8 +1045,8 @@ int gdb_serial_stub(struct kgdb_state *ks)
 				gdb_cmd_detachkill(ks);
 				return DBG_PASS_EVENT;
 			}
-#endif
 			fallthrough;
+#endif
 		case 'C': /* Exception passing */
 			tmp = gdb_cmd_exception_pass(ks);
 			if (tmp > 0)