gnu: syncthing-gtk: Wrap with GUIX_GDK_PIXBUF_MODULE_FILES.

Fixes #1266.

Although currently happening, previously, syncthing-gtk would fail to open
with "Type RsvgHandle has already been registered".  This was likely due to
various grafts as different librsvg would appear in environment variables
compared to syncthing-gtk references.  This change fixed it at the time and
should help void it in the future.

* gnu/packages/syncthing.scm (syncthing-gtk)[arguments]{phases}: In wrap-libs
phase, also wrap with GUIX_GDK_PIXBUF_MODULE_FILES.  Change GI_TYPELIB_PATH to
use suffix.

Change-Id: I2ae2e2ace5df5882b1cbdcd149cc0bc4823d3939

gnu/packages/syncthing.scm

@@ -184,7 +184,9 @@ Protocol.")
              (lambda* (#:key outputs #:allow-other-keys)
                (let ((out (assoc-ref outputs "out")))
                  (wrap-program (string-append out "/bin/syncthing-gtk")
-                   `("GI_TYPELIB_PATH" ":" prefix
+                   `("GUIX_GDK_PIXBUF_MODULE_FILES" ":" suffix
+                     (,(getenv "GUIX_GDK_PIXBUF_MODULE_FILES")))
+                   `("GI_TYPELIB_PATH" ":" suffix
                      (,(getenv "GI_TYPELIB_PATH"))))))))))
       (inputs
        (list bash-minimal