[CIFS] Fix build warning

Fix build warning in Shirish's recent SMB3 signing patch
which occurs when SMB2 support is disabled in Kconfig.

fs/built-in.o: In function `cifs_setup_session':
>> (.text+0xa1767): undefined reference to `generate_smb3signingkey'

Pointed out by: automated 0-DAY kernel build testing backend
Intel Open Source Technology Center

CC: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/smb2ops.c

@@ -709,6 +709,7 @@ struct smb_version_operations smb30_operations = {
 	.get_lease_key = smb2_get_lease_key,
 	.set_lease_key = smb2_set_lease_key,
 	.new_lease_key = smb2_new_lease_key,
+	.generate_signingkey = generate_smb3signingkey,
 	.calc_signature = smb3_calc_signature,
 };