Fix warning when using key parameter of kmip_bio_register_symmetric_key

[ldonoso]

Problem:

clang 10 shows the following warning:

/libkmip/libkmip/src/kmip_bio.c:313:14:
warning: assigning to 'uint8 *' (aka 'unsigned char *') from 'char *'
converts between pointers to integer types with different sign
[-Wpointer-sign]
    bs.value = key;
             ^ ~~~

Solution:

Change the type of the parameter so it matches the requested type.