gdb/objc: fix bug in objc_language::opcode_print_table

In this commit:

  commit b7c6e27d
  Date:   Tue Aug 4 17:07:59 2020 +0100

      gdb: Convert language_data::la_op_print_tab to a method

A bug was introduced, the objc language now returns the wrong op_print
table.  Fixed in this commit.

gdb/ChangeLog:

	* objc-lang.c (objc_language::opcode_print_table): Return
	objc_op_print_tab.