abi_check: Update submodules
abi_check: Update submodules

When grabbing a fresh copy of a branch, it's required to also fetch the
submodule. Add fetching the submodule to abi_check.py.

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls