subprojects: Add valijson wrap

Addresses the following meson warning:

```
Has header "valijson/validator.hpp" : NO
Cloning into 'valijson'...
remote: Enumerating objects: 4827, done.
remote: Counti

subprojects: Add valijson wrap

Addresses the following meson warning:

```
Has header "valijson/validator.hpp" : NO
Cloning into 'valijson'...
remote: Enumerating objects: 4827, done.
remote: Counting objects: 100% (1766/1766), done.
remote: Compressing objects: 100% (893/893), done.
remote: Total 4827 (delta 875), reused 1685 (delta 832), pack-reused 3061
Receiving objects: 100% (4827/4827), 4.80 MiB | 3.60 MiB/s, done.
Resolving deltas: 100% (2637/2637), done.
HEAD is now at 9183462118f5 Fix return type on getMinLength
Subproject exists but has no meson.build file.
Subproject valijson is buildable: NO (disabling)
meson.build:87: WARNING: include_directories sandbox violation!
The project is trying to access the directory 'subprojects/valijson/include' which belongs to a different
subproject. This is a problem as it hardcodes the relative paths of these two projects.
This makes it impossible to compile the project in any other directory layout and also
prevents the subproject from changing its own directory layout.

Instead of poking directly at the internals the subproject should be executed and
it should set a variable that the caller can then use. Something like:

some_dep = declare_dependency(include_directories: include_directories('include'))

[provide]
some = some_dep

some_dep = dependency('some')
executable(..., dependencies: [some_dep])

This warning will become a hard error in a future Meson release.
```

Change-Id: I26b4ced45285943b6cae1c1feeace2f853931b0d
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

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