proftpd: remove UseIPv6 option from sample config if built without IPv6

proftpd refuses to start if it finds a UseIPv6 option in the configuration
file (even if set to off) when built without IPv6 support, so comment out
line.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/proftpd/proftpd.mk

@@ -31,6 +31,7 @@ define PROFTPD_INSTALL_TARGET_CMDS
 	$(INSTALL) -D -m 0755 $(@D)/proftpd $(TARGET_DIR)/usr/sbin/proftpd
 	@if [ ! -f $(TARGET_DIR)/etc/proftpd.conf ]; then \
 		$(INSTALL) -m 0644 -D $(@D)/sample-configurations/basic.conf $(TARGET_DIR)/etc/proftpd.conf; \
+		$(if $(BR2_INET_IPV6),,$(SED) 's/^UseIPv6/# UseIPv6/' $(TARGET_DIR)/etc/proftpd.conf;) \
 	fi
 	$(INSTALL) -m 0755 package/proftpd/S50proftpd $(TARGET_DIR)/etc/init.d
 endef