[gtk/ebassi/gidocgen] build: Add missing variable definition

[Emmanuele Bassi <ebassi src gnome org>]

commit ca3739d94bf0ae234ee9678116e6333ca68f9ce0
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Fri Mar 5 16:44:17 2021 +0000

    build: Add missing variable definition

 gtk/meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/meson.build b/gtk/meson.build
index 90d7b277a9..fe0ec456d9 100644
--- a/gtk/meson.build
+++ b/gtk/meson.build
@@ -778,6 +778,7 @@ if os_unix
 endif
 
 if os_win32
+  gtk_win32_print_headers = []
   gtk_win32_print_sources = files([
     'gtkprint-win32.c',
     'gtkprintoperation-win32.c',