libxl: drop stray const from function return type

Some compiler versions warn about this, causing the build to fail due
to -Werror.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -5354,8 +5354,8 @@ libxl_numainfo *libxl_get_numainfo(libxl
     return ret;
 }
 
-static const int libxl__xc_version_wrap(libxl__gc *gc, libxl_version_info *info,
-                                        xen_build_id_t *build)
+static int libxl__xc_version_wrap(libxl__gc *gc, libxl_version_info *info,
+                                  xen_build_id_t *build)
 {
     int r;