[gimp] meson: add a `install-extensions` target to meson/ninja.

[Jehan <jehanp src gnome org>]

commit a3498f427af25f8b8bbc40fff1246fc2b559c95e
Author: Jehan <jehan girinstud io>
Date:   Sun Oct 18 01:13:42 2020 +0200

    meson: add a `install-extensions` target to meson/ninja.

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index 3f9e9a12d9..08caeb0e8f 100644
--- a/meson.build
+++ b/meson.build
@@ -1718,6 +1718,7 @@ foreach dir : [
     { 'dir': 'libgimpwidgets', 'deps': libgimpwidgets},
     { 'dir': 'icons' },
     { 'dir': 'plug-ins', },
+    { 'dir': 'extensions', },
   ]
   run_target('install-' + dir.get('dir'),
     command: [