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: e858ef1 ("selftests: Add l2tp tests")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: David Ahern <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

Author: dsahern