Nettle switched prototypes for base64_encode_raw() as follows:
-base64_encode_raw(uint8_t *dst, size_t length, const uint8_t *src);
+base64_encode_raw(char *dst, size_t length, const uint8_t *src);

That means we need to cast fist param to void if we want to avoid
warnings on different platforms.
Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>