tests: net: lwm2m: Fix zsock_fcntl impl name

The syscall name has _impl postfix so adjusting the stub
name.

zephyr-sdk-0.16.5-1/x86_64-zephyr-elf/bin/../lib/gcc/
  x86_64-zephyr-elf/12.2.0/../../../../x86_64-zephyr-elf/bin/ld.bfd:
  app/libapp.a(lwm2m_engine.c.obj): in function `zsock_fcntl_impl':
.../syscalls/socket.h:363: undefined reference to `z_impl_zsock_fcntl_impl'

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>