MINOR: sample: Move aes_gcm_dec implementation into sample.c

MINOR: sample: Move aes_gcm_dec implementation into sample.c

aes_gcm_dec is independent of the TLS implementation and fits better
in sample.c file with others hash functions.

[Cf: I slightly updated this patch to move aes_gcm_dec converter in sample.c
     instead the new file crypto.c]

Reviewed-by: Tim Duesterhus <tim@bastelstu.be>