ima: allow users to specify the pcr index with IMA_MEASURE_PCR_IDX

The IMA_MEASURE_PCR_IDX option is currently not visible in the kconfig
frontend, so it always uses its default, 10. This means that the
'range 8 14' is dead code, and users are unable to specify the pcr index
value.

In a previous discussion, Mimi explained that users should be able to use
this config option to specify the pcr index. [1]

Let's add a prompt for users to specify the pcr index, when EXPERT is
enabled.

This dead range was found by kconfirm, a static analysis tool for Kconfig.

Link: https://lore.kernel.org/all/1feff118-4afa-4b9c-86f1-271a7a88208f@gmail.com/T/#mc4efa2491b4937eb7c9e532c29ffba516a70e662 [1]
Signed-off-by: Julian Braha <julianbraha@gmail.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>