UserspaceEmulator: downgrade TODO to dbgln for invalid fcntl cmd arg

Author: https://github.com/bcoles
Commit: 7156b61d57
Pull-request: https://github.com/SerenityOS/serenity/pull/5801
Reviewed-by: https://github.com/awesomekling

Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp

@@ -668,7 +668,7 @@ u32 Emulator::virt$fcntl(int fd, int cmd, u32 arg)
     case F_ISTTY:
         break;
     default:
-        TODO();
+        dbgln("Invalid fcntl cmd: {}", cmd);
     }
 
     return syscall(SC_fcntl, fd, cmd, arg);