Fix type_map()

[kleisauke]

We cannot declare VipsTypeMap2Fn as opaque as this callback is used by type_map(). Instead, restore the VipsTypeMap2Fn definition and use an opaque pointer for the second argument of vips_type_map().

Details

$ python3 gen_cpp_binding.py
Traceback (most recent call last):
  File "/home/kleisauke/wasm-vips/build/gen_cpp_binding.py", line 295, in <module>
    generate_enums_flags('enums.cpp')
  File "/home/kleisauke/wasm-vips/build/gen_cpp_binding.py", line 230, in generate_enums_flags
    type_map(type_from_name('GEnum'), add_enum)
  File "/home/kleisauke/.local/lib/python3.12/site-packages/pyvips/base.py", line 105, in type_map
    cb = ffi.callback('VipsTypeMap2Fn', fn)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected a function ctype, got 'VipsTypeMap2Fn'

Regressed since: #464.