[Linux] Add 1.8.8 to appdata.xml

Author: facchinm

build/linux/dist/appdata.xml

@@ -36,6 +36,7 @@
   <content_rating type="oars-1.1" />
 
   <releases>
+    <release date="2018-12-06" version="1.8.8"/>
     <release date="2018-09-11" version="1.8.7"/>
     <release date="2018-08-23" version="1.8.6"/>
     <release date="2017-10-02" version="1.8.5"/>