Fix gcc Werror because of a typo in a switch/case explicit fallthrough comment

GCC crash because of -Werror=implicit-fallthrough. However the fallthrough exists but has a typo.

Thanks for omac777 analysis (7d0b8155a3)

However the fallthrough exists but has a typo

(cherry picked from commit ea0c714139)

qrexec-lib/pack.c

@@ -100,7 +100,7 @@ void wait_for_result(void)
                      * desynchronized in this case */
                     return;
                 }
-                /* fall though */
+		/* fallthrough */
             default:
                 call_error_handler("File copy: %s%s%s",
                         strerror(hdr.error_code), last_filename_prefix, last_filename);