package/brltty: add expat optional dependency

expat optional dependency has been added in version 6.0 with
https://github.com/brltty/brltty/commit/a37febee8cc4215eaac668a064ebc590684122b7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/brltty/brltty.mk

@@ -50,6 +50,13 @@ else
 BRLTTY_CONF_OPTS += --without-espeak
 endif
 
+ifeq ($(BR2_PACKAGE_EXPAT),y)
+BRLTTY_DEPENDENCIES += expat
+BRLTTY_CONF_OPTS += --enable-expat
+else
+BRLTTY_CONF_OPTS += --disable-expat
+endif
+
 ifeq ($(BR2_PACKAGE_FLITE),y)
 BRLTTY_DEPENDENCIES += flite
 BRLTTY_CONF_OPTS += --with-flite=$(STAGING_DIR)/usr