package/s6-networking: fix dependency when libressl is enabled

Commit "c5b85231fb s6-networking: enable SSL if libressl is selected"
actually dropped the dependency on s6-dns and s6 when libressl is
enabled.
Fix this by using += inside the conditional code.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 30ff614a035dd49e357f7c3560555dae8e605f80)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/s6-networking/s6-networking.mk

@@ -25,7 +25,7 @@ S6_NETWORKING_CONF_OPTS = \
 
 ifeq ($(BR2_PACKAGE_LIBRESSL),y)
 S6_NETWORKING_CONF_OPTS += --enable-ssl=libressl
-S6_NETWORKING_DEPENDENCIES = libressl
+S6_NETWORKING_DEPENDENCIES += libressl
 endif
 
 define S6_NETWORKING_CONFIGURE_CMDS