Extract callback type aliases into a GC-allowed block.

It turns out, it's completely OK for a `@nogc` to receive a GC-allowed
function as an argument, you just can't call it. Passing it to some
other routine is fine as well.

Moreover, it's not forbidden to pass a `@nogc` function where a
GC-allowed one is expected. So the only thing that needs to be done is
to put the callback type aliases into a block without `@nogc`, so they
are not marked as such.

Author: Michail Pevnev

source/deimos/ncurses/curses.d

@@ -925,8 +925,6 @@ int vid_attr(chtype a, ...)
  * These functions are extensions - not in X/Open Curses.
  */
  //TODO Check
-alias int function(WINDOW *, void *) NCURSES_WINDOW_CB;
-alias int function(SCREEN *, void *) NCURSES_SCREEN_CB;
 
 bool is_term_resized(int lines, int columns);
 char* keybound(int keycode, int count);
@@ -1492,3 +1490,12 @@ body
 }
 
 }//end extern (C)
+
+// Things where @nogc would be needlessly limiting.
+extern (C) nothrow
+{
+
+alias int function(WINDOW *, void *) NCURSES_WINDOW_CB;
+alias int function(SCREEN *, void *) NCURSES_SCREEN_CB;
+
+}