usign-exec: change usign_f_* fingerprint argument to char[17]

usign-exec: change usign_f_* fingerprint argument to char[17]

This makes it more obvious that a buffer with space for 17 characters is
expected to be passed. The code still works the same (a char[17] is
equivalent to char* as an argument).

Signed-off-by: Matthias Schiffer <[email protected]>