pixman: add missing MesonPackage

[wdconinc]

This PR fixes up #47529 which forgot to add MesonPackage.

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[haampie]

It also forgets to import spack.build_systems.{autotools,meson}

[wdconinc]

It also forgets to import spack.build_systems.{autotools,meson}

Many packages do not import this. I am not sure if we can insist on this broadly without an audit or mypy check that points this out.

[haampie]

Many packages do not import this. I am not sure if we can insist on this broadly without an audit or mypy check that points this out.

Feel free to merge #47619 ;)

[wdconinc]

It also forgets to import spack.build_systems.{autotools,meson}

Deferred to #47617.