Update mutability of f's index

Author: osa1

test/ld/ok/fun-ptr.linked.wat.ok

@@ -21,6 +21,6 @@
   (table (;0;) 1 1 funcref)
   (memory (;0;) 2)
   (global (;0;) i32 (i32.const 65536))
-  (global (;1;) i32 (i32.const 0))
+  (global (;1;) (mut i32) (i32.const 0))
   (start $link_start)
   (elem (i32.const 0) func $f))