package/proftpd: add libidn2 optional dependency

libidn2 is an optional dependency which is enabled by default since
version 1.3.8 and
https://github.com/proftpd/proftpd/commit/5f61ac028e0930e7de9ea2a00864ccb6dc048f64

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/proftpd/proftpd.mk

@@ -27,6 +27,10 @@ PROFTPD_CONF_OPTS = \
 	--with-gnu-ld \
 	--without-openssl-cmdline
 
+ifeq ($(BR2_PACKAGE_LIBIDN2),y)
+PROFTPD_DEPENDENCIES += libidn2
+endif
+
 ifeq ($(BR2_PACKAGE_PROFTPD_MOD_CAP),y)
 PROFTPD_CONF_OPTS += --enable-cap
 PROFTPD_DEPENDENCIES += libcap