patch 8.1.1480: desktop file check doesn't run on CI

Problem:    Desktop file check doesn't run on CI.
Solution:   Install the desktip-file-utils packages. (Christian Brabandt,
            closes #4498)

.travis.yml

@@ -86,6 +86,7 @@ addons:
       - tcl-dev
       - cscope
       - libgtk2.0-dev
+      - desktop-file-utils
   homebrew:
     packages:
       - lua

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1480,
 /**/
     1479,
 /**/