selftests: net: change shebang to bash in amt.sh

amt.sh is written in bash, not sh.
So, shebang should be bash.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>