Change quotes around GTK headers for angle brackets

The linux buildbots have started to fail compilation due to not
finding the gtk headers. The quotes have been changed to angle brackets
to indicate to the compiler that these are system includes.

modules/highgui/src/window_gtk.cpp

@@ -45,8 +45,8 @@
 
 #if defined (HAVE_GTK)
 
-#include "gtk/gtk.h"
-#include "gdk/gdkkeysyms.h"
+#include <gtk/gtk.h>
+#include <gdk/gdkkeysyms.h>
 #include <gdk-pixbuf/gdk-pixbuf.h>
 #include <stdio.h>