meson: adjust nlohmann-json dependency

- Simplify the nlohmann-json dependency logic in meson.build
- Change wrap file name to align with other repositories.
- Use HEAD for nlohmann-json revision ra

meson: adjust nlohmann-json dependency

- Simplify the nlohmann-json dependency logic in meson.build
- Change wrap file name to align with other repositories.
- Use HEAD for nlohmann-json revision rather than a pinned version.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5ba6cc684c7c2dc8889e9a4cd882d5c6840b4b92

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