meson.build: bump Meson minimum version to 0.59.0
When creating a new build directory, Meson prints a warning:
WARNING: Project specifies a minimum meson_version '>= 0.56.0' but uses features which were added in newer versions:
- 0.59.0: {'unescaped_variables arg in pkgconfig.generate'}
Since 0.59.0 was released a while ago and no one has complained so far, it should be reasonable to just bump the minimum version required, rather than try to make it 0.56.0 compliant.