The header-only package cannot be included in the SDK without marking
the main package with ALLOW_EMPTY.
Fixes rootfs problem:
```
The following packages have unmet dependencies:
imx-gpu-sdk : Depends: nlohmann-json but it is not installable
E: Unable to correct problems, you have held broken packages.
```
Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
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