[balsa/oauth2-support] Fix build error when using clang

[Albrecht Dreß <albrecht src gnome org>]

commit 37c4f12528fe8c045b780fbb4d277e084acdc887
Author: Albrecht Dreß <albrecht dress arcor de>
Date:   Fri Jan 8 16:04:46 2021 +0100

    Fix build error when using clang
    
    Signed-off-by: Albrecht Dreß <albrecht dress arcor de>

 libnetclient/net-client-utils.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libnetclient/net-client-utils.c b/libnetclient/net-client-utils.c
index 46b914dc5..fe1042432 100644
--- a/libnetclient/net-client-utils.c
+++ b/libnetclient/net-client-utils.c
@@ -391,7 +391,7 @@ net_client_auth_oauth2_calc(const gchar *user, gboolean rfc7628mode, NetClient *
                buffer = g_strdup_printf("user=%s\001auth=Bearer %s\001\001", user, access_token);
        }
        g_debug("%s: '%s'", __func__, buffer);
-       result = g_base64_encode(buffer, strlen(buffer));
+       result = g_base64_encode((const guchar *) buffer, strlen(buffer));
        g_free(buffer);
        return result;
 }