gzip: Move cleanup to separate function

to avoid exposing gzip function into wrapper.c

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 00fce9cb6c)

include/libssh/crypto.h

@@ -227,6 +227,8 @@ int secure_memcmp(const void *s1, const void *s2, size_t n);
 ENGINE *pki_get_engine(void);
 #endif /* HAVE_LIBCRYPTO */
 
+void compress_cleanup(struct ssh_crypto_struct *crypto);
+
 #ifdef __cplusplus
 }
 #endif