Add tiffdiff build target to meson

[Copilot]

tiffdiff was built by CMake and MSVC project files but had no meson equivalent, leaving meson builds without the tool.

Changes

• utils/tificc/meson.build: Added tiffdiff executable target using the existing tifdiff.c source with liblcms2_dep, common_dep, and tiff_dep dependencies — mirrors the cmake definition in Lcms2Tools.cmake

tiffdiff_exe = executable(
  'tiffdiff',
  files('tifdiff.c'),
  dependencies: [liblcms2_dep, common_dep, tiff_dep],
  install: true,
)

The target is only built when tiff_dep is found, consistent with how tificc is conditionally included in utils/meson.build.