clipboard: add `[heap]` tag to the Clipboard structs (#14412)

vlib/clipboard/clipboard_darwin.c.v

@@ -8,6 +8,7 @@ module clipboard
 // Clipboard represents a system clipboard.
 //
 // System "copy" and "paste" actions utilize the clipboard for temporary storage.
+[heap]
 pub struct Clipboard {
 	pb             voidptr
 	last_cb_serial i64

vlib/clipboard/clipboard_windows.c.v

@@ -54,6 +54,7 @@ fn C.DestroyWindow(hwnd C.HWND)
 // Clipboard represents a system clipboard.
 //
 // System "copy" and "paste" actions utilize the clipboard for temporary storage.
+[heap]
 struct Clipboard {
 	max_retries int
 	retry_delay int

vlib/clipboard/dummy/dummy_clipboard.v

@@ -3,6 +3,7 @@ module dummy
 // Clipboard represents a system clipboard.
 //
 // System "copy" and "paste" actions utilize the clipboard for temporary storage.
+[heap]
 pub struct Clipboard {
 mut:
 	text     string // text data sent or received

vlib/clipboard/x11/clipboard.c.v

@@ -138,6 +138,7 @@ enum AtomType {
 	text_html = 9
 }
 
+[heap]
 pub struct Clipboard {
 	display &C.Display
 mut: