fix: Add missing block_cipher.c to pico_mbedtls

[kripton]

Fixes #2882

It looks like the reference to block_cipher.c was missed in pico_mbedtls.

I did NOT recompile any examples with this changes but the file exists on disk and will do nothing if the relevant #define is not set in mbedtls_config.h

[kripton]

OK, I need to rework this. Merge conflict arose because I did the changes on main but switched the base branch of this PR to develop as instructed

[kripton]

Ok, commit updated against latest develop. I also checked https://github.com/raspberrypi/pico-sdk/blob/develop/src/rp2_common/pico_mbedtls/mbedtls.BUILD#L8 but it shouldn't be an issue here since a glob pattern is used.

[peterharperuk]

I think support for this was only added in 3.6.0? In which case we should make an effort to still build with older versions of mbedtls.

[kripton]

I think support for this was only added in 3.6.0? In which case we should make an effort to still build with older versions of mbedtls.

Agreed, that makes sense. I changed the PR accordingly and tested my project (without the workaround described in #2882) compiles fine. However, I have not tested using an older version of mbedtls (<3.6.0)