util/grub-install-common: Fix the key of the --core-compress option

Commit f23bc6510 (Transform -C option to grub-mkstandalone to
--core-compress available in all grub-install flavours.) declared
a new long option for specifying the compression method to use for
the core image.

However, the option key has not been replaced in the parser function,
it still expects the old one formerly used by grub-mkstandalone.
Because of this the option is not recognized by any of the utils for
which it is listed as supported.

Signed-off-by: Ákos Nagy <nagyakos@outlook.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub-install-common.c

@@ -472,7 +472,7 @@ grub_install_parse (int key, char *arg)
 {
   switch (key)
     {
-    case 'C':
+    case GRUB_INSTALL_OPTIONS_INSTALL_CORE_COMPRESS:
       if (grub_strcmp (arg, "xz") == 0)
 	{
 #ifdef HAVE_LIBLZMA