[gimp] INSTALL: add xdg-desktop-portal in the runtime dependencies.

[Jehan <jehanp src gnome org>]

commit bea061888399eadd2a111061048711d530741dbf
Author: Jehan <jehan girinstud io>
Date:   Sun Jan 2 19:48:32 2022 +0100

    INSTALL: add xdg-desktop-portal in the runtime dependencies.
    
    Cf. #1074 for color-picking, as well as the screenshot plug-in.

 INSTALL.in | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/INSTALL.in b/INSTALL.in
index 9a9bc30f4c..a07f6b1b8f 100644
--- a/INSTALL.in
+++ b/INSTALL.in
@@ -221,6 +221,7 @@ header files installed.
      GJS for Javascript plug-ins
      LuaJIT and LGI for Lua plug-ins
      dot for "Show Image Graph" (unstable branches)
+     xdg-desktop-portal implemented for your desktop for various D-Bus API (screenshot, color-picking…)