[gimp] meson: fix the build.

[Jehan <jehanp src gnome org>]

commit ee2a3104e9eb91a05d52191260bc2d882ace7eb0
Author: Jehan <jehan girinstud io>
Date:   Wed May 19 23:03:46 2021 +0200

    meson: fix the build.
    
    Commit a8f020d889 broke the build. I removed a subdir() by mistake.

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index cf1e3e8463..7222ec61d7 100644
--- a/meson.build
+++ b/meson.build
@@ -1610,6 +1610,7 @@ meson.add_dist_script('meson_dist_script.sh')
 rootInclude = include_directories('.')
 appInclude  = include_directories('app')
 
+subdir('build/windows')
 if get_option('windows-installer')
   subdir('po-windows-installer')
   subdir('build/windows/installer')