[gimp] Re-enable Twain plugin on Windows for Meson build

[Jehan <jehanp src gnome org>]

commit 783f7cae5527c0de795d2a0fb29162ccf769faa5
Author: Félix Piédallu <fpiedallu kalrayinc com>
Date:   Tue Jul 28 17:20:16 2020 +0200

    Re-enable Twain plugin on Windows for Meson build

 plug-ins/meson.build       | 2 +-
 plug-ins/twain/meson.build | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)
---
diff --git a/plug-ins/meson.build b/plug-ins/meson.build
index 4aeb846420..2e60a31550 100644
--- a/plug-ins/meson.build
+++ b/plug-ins/meson.build
@@ -31,5 +31,5 @@ subdir('python')
 subdir('screenshot')
 subdir('script-fu')
 subdir('selection-to-path')
-# subdir('twain')
+subdir('twain')
 subdir('ui')
diff --git a/plug-ins/twain/meson.build b/plug-ins/twain/meson.build
index 6229461a35..bf75f25457 100644
--- a/plug-ins/twain/meson.build
+++ b/plug-ins/twain/meson.build
@@ -1,4 +1,6 @@
-if platform_windows
+if not platform_windows
+  subdir_done()
+endif
 
 plugin_name = 'twain'
 
@@ -31,5 +33,3 @@ twain = executable(plugin_name,
   install: true,
   install_dir: gimpplugindir / 'plug-ins' / plugin_name,
 )
-
-endif