vl: Move DisplayType typedef to vl.c

Now the type is only used inside vl.c and doesn't need to be in a
header file.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

vl.c

@@ -2077,6 +2077,15 @@ static void select_vgahw(const char *p)
     }
 }
 
+typedef enum DisplayType {
+    DT_DEFAULT,
+    DT_CURSES,
+    DT_SDL,
+    DT_COCOA,
+    DT_GTK,
+    DT_NONE,
+} DisplayType;
+
 static DisplayType select_display(const char *p)
 {
     const char *opts;