gnu: mate-themes: Update source hash.

This is a followup to commit 44350d23da.

* gnu/packages/mate.scm (mate-themes)[source]: Update hash.
* gnu/packages/gtk.scm (gtk+): Add reminder comment.

gnu/packages/gtk.scm

@@ -649,6 +649,8 @@ application suites.")
 (define-public gtk+
   (package (inherit gtk+-2)
    (name "gtk+")
+   ;; NOTE: When updating the version of 'gtk+', the hash of 'mate-themes' in
+   ;;       mate.scm will also need to be updated.
    (version "3.22.6")
    (source (origin
             (method url-fetch)

gnu/packages/mate.scm

@@ -70,7 +70,7 @@
                                   ".tar.xz"))
               (sha256
                (base32
-                "12p6xvqs8smbk9nivi43089fiw1jbb3g9836arr0sksmmziklnvy"))))
+                "1gsfzrcbwp7835pbilk7cvda8hjsf9g3gl4llbm61y9j7a4x2kn6"))))
     (build-system gnu-build-system)
     (native-inputs
      `(("pkg-config" ,pkg-config)