selftests: firmware: add CONFIG_FW_LOADER_USER_HELPER_FALLBACK to config

CONFIG_FW_LOADER_USER_HELPER_FALLBACK=y is required for fw_fallback.sh.
Without it, fw_fallback.sh fails with 'usermode helper disabled so
ignoring test'. Enable the config in selftest so that it gets built by
default.

Signed-off-by: Dan Rue <dan.rue@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>