SPECS: libxmlb: Align stemmer build condition

[Jingwiw]

Changes

• Add the missing remote asset checksum and use %autochangelog directly.

• Make the existing stemmer bcond drive the Meson option.

  Upstream defaults stemmer to false, so the default build remains unchanged. This makes %{with stemmer} actually enable the feature that already has a conditional BuildRequires.

  Source: meson_options.txt

  option('stemmer', type : 'boolean', value : false, description : 'enable stemmer support')

  Source: meson.build

  if get_option('stemmer')
    cc = meson.get_compiler('c')
    stemmer = cc.find_library('stemmer')
    libxmlb_deps += stemmer
    conf.set('HAVE_LIBSTEMMER', 1)
  endif

• Use pkgconfig(gio-2.0) instead of the broader glib-devel package name.

  libxmlb asks Meson for gio-2.0, so the BuildRequires should name the provider interface it actually consumes.

  Source: meson.build

  gio = dependency('gio-2.0', version : '>= 2.45.8')

AIGC Declaration: CodeX with gpt5.5 was used as coding agent.

Signed-off-by: Jingwiw wangjingwei@iscas.ac.cn