selftests: Make l2tp.sh executable

Kernel test robot reported that the l2tp.sh test script failed:
    # selftests: net: l2tp.sh
    # Warning: file l2tp.sh is not executable, correct this.

Set executable bits.

Fixes: e858ef1c ("selftests: Add l2tp tests")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>