flatpak: Use fallback-x11

From Flatpak's NEWS:

> A new fallback-x11 permission grants X11 access, but only if the user
> is running in a X11 session. For applications that support both
> Wayland and X11, this can be used to ensure that the app doesn't have
> unnecessary X11 access while in Wayland, but still works in an X11
> session.

build/flatpak/org.gimp.GIMP-nightly.json

@@ -9,7 +9,7 @@
     "rename-desktop-file": "gimp.desktop",
     "rename-icon": "gimp",
     "finish-args": ["--share=ipc", "--share=network",
-                    "--socket=x11", "--socket=wayland",
+                    "--socket=fallback-x11", "--socket=wayland",
                     "--device=dri",
                     "--filesystem=host", "--filesystem=xdg-config/GIMP",
                     "--filesystem=xdg-config/gtk-3.0", "--filesystem=/tmp",