rofi: Change vim:ft to vim:syn

Setting the file type of a file is different than simply changing its
syntax. In this case, the rofi config was treated as a real .java file.
By setting syntax explicitly, we avoid re-declaring the filetype.

.config/rofi/config.rasi

@@ -15,4 +15,4 @@ configuration {
   display-calc: "Calc";
 }
 
-// vim:ft=java
+// vim:syn=java