xen: fix build when !CONFIG_GRANT_TABLE

Move struct grant_table; in grant_table.h above
ifdef CONFIG_GRANT_TABLE. This fixes the following:

/build/xen/include/xen/grant_table.h:84:50: error: 'struct grant_table'
declared inside parameter list will not be visible outside of this
definition or declaration [-Werror]
   84 | static inline int mem_sharing_gref_to_gfn(struct grant_table *gt,
      |

Signed-off-by: Connor Davis <connojdavis@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/xen/grant_table.h

@@ -28,9 +28,10 @@
 #include <public/grant_table.h>
 #include <asm/grant_table.h>
 
-#ifdef CONFIG_GRANT_TABLE
 struct grant_table;
 
+#ifdef CONFIG_GRANT_TABLE
+
 extern unsigned int opt_max_grant_frames;
 
 /* Create/destroy per-domain grant table context. */