opcodes/m68k: enable libopcodes styling for GDB

The following commit added libopcodes styling for m68k:

  commit c22ff449275c91e4842bb10c650e83c572580f65
  Date:   Tue Feb 14 18:07:19 2023 +0100

      opcodes: style m68k disassembler output

but didn't set disassemble_info::created_styled_output in
disassemble.c, which is needed in order for GDB to start using the
libopcodes based styling.

This commit fixes this small oversight.  GDB now styles correctly.