[gnome-shell.modules] Work around problem of git with 'gtk+'

Use gtk%2b in the checkout URL, since recent versions of GIT have
trouble with the '+' in gtk+.

https://bugzilla.gnome.org/show_bug.cgi?id=625135

tools/build/gnome-shell.modules

@@ -85,7 +85,7 @@
   </autotools>
 
   <autotools id="gtk3">
-    <branch checkoutdir="gtk3" repo="git.gnome.org" module="gtk+"/>
+    <branch checkoutdir="gtk3" repo="git.gnome.org" module="gtk%2b"/>
     <dependencies>
       <dep package="glib"/>
       <dep package="pango"/>