package/lftp: fix build with host expat

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=11881

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit d8011a09f741cde3e1c48edb637b91f31129a04b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/lftp/lftp.mk

@@ -25,6 +25,9 @@ endif
 
 ifeq ($(BR2_PACKAGE_EXPAT)$(BR2_PACKAGE_LFTP_PROTO_HTTP),yy)
 LFTP_DEPENDENCIES += expat
+LFTP_CONF_OPTS += --with-expat=$(STAGING_DIR)/usr
+else
+LFTP_CONF_OPTS += --without-expat
 endif
 
 ifeq ($(BR2_PACKAGE_GNUTLS),y)