patch 8.0.1708: mkdir with 'p' flag fails on existing directory

Problem:    Mkdir with 'p' flag fails on existing directory, which is
            different from the mkdir shell command.
Solution:   Don't fail if the directory already exists. (James McCoy,
            closes #2775)

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1708,
 /**/
     1707,
 /**/