gnu: guile-syntax-highlight: Move Gitile variant to guile-xyz.scm.

Fixes a regression introduced in
5dac09e263 whereby version-control.scm
would inherit from GUILE-SYNTAX-HIGHLIGHT, which is in guile-xyz.scm,
thereby leading to unbound variable errors.

Reported by vivien on #guix.

* gnu/packages/version-control.scm (guile-syntax-highlight-for-gitile):
Move to...
* gnu/packages/guile-xyz.scm (guile-syntax-highlight-for-gitile):
... here.  New variable.  Add 'properties' field.

gnu/packages/guile-xyz.scm

@@ -755,6 +755,29 @@ HTML (via SXML) or any other format for rendering.")
     (home-page "https://dthompson.us/projects/guile-syntax-highlight.html")
     (license license:lgpl3+)))
 
+;; gitile requires a more recent version than the latest release.
+(define-public guile-syntax-highlight-for-gitile
+  (let ((commit "897fa5156ff41588e0d281eb00e4e94de63ccd8a")
+        (revision "0"))
+    (package
+      (inherit guile-syntax-highlight)
+      (version (git-version "0.1" revision commit))
+      (source (origin
+                (method git-fetch)
+                (uri (git-reference
+                       (url "https://git.dthompson.us/guile-syntax-highlight.git")
+                       (commit commit)))
+                (file-name (git-file-name "guile-syntax-highlight" version))
+                (sha256
+                 (base32
+                  "18zlg4mkgd3swgv2ggfz91ivnnzc0zhvc9ybgrxg1y762va9hyvj"))))
+      (native-inputs
+       `(("autoconf" ,autoconf)
+         ("automake" ,automake)
+         ("texinfo" ,texinfo)
+         ,@(package-native-inputs guile-syntax-highlight)))
+      (properties '((hidden? . #t))))))
+
 (define-public guile2.2-syntax-highlight
   (package
     (inherit guile-syntax-highlight)

gnu/packages/version-control.scm

@@ -1527,28 +1527,6 @@ also walk each side of a merge and test those changes individually.")
 control to Git repositories.")
     (license license:gpl2)))
 
-;; gitile requires a more recent version than the latest release.
-(define guile-syntax-highlight-for-gitile
-  (let ((commit "897fa5156ff41588e0d281eb00e4e94de63ccd8a")
-        (revision "0"))
-    (package
-      (inherit guile-syntax-highlight)
-      (version (git-version "0.1" revision commit))
-      (source (origin
-                (method git-fetch)
-                (uri (git-reference
-                       (url "https://git.dthompson.us/guile-syntax-highlight.git")
-                       (commit commit)))
-                (file-name (git-file-name "guile-syntax-highlight" version))
-                (sha256
-                 (base32
-                  "18zlg4mkgd3swgv2ggfz91ivnnzc0zhvc9ybgrxg1y762va9hyvj"))))
-      (native-inputs
-       `(("autoconf" ,autoconf)
-         ("automake" ,automake)
-         ("texinfo" ,texinfo)
-         ,@(package-native-inputs guile-syntax-highlight))))))
-
 (define-public gitile
   (package
     (name "gitile")