USBGuard cannot be compiled with GCC 4.8.x

[dkopecek]

The https://github.com/nlohmann/json library lacks support for GCC 4.8.x. Details: https://github.com/nlohmann/json#supported-compilers

This is a problem because we would like to compile USBGuard for CentOS/RHEL 7.

Possible solutions:

1. support gcc 4.8.1 nlohmann/json#212
2. Switch to a different json library
3. Support another library in parallel (high maintenance cost)

[dkopecek]

It seems that all json packages available on CentOS/RHEL 7 are unusable. json-c and jansson are not thread-safe and json-glib has dependencies on other glib libraries which we use in case the dbus component is enabled but I would like to avoid them in the core.

Switching to protocol buffers (protobuf) seem like a viable option.

[dkopecek]

Resolved in 86c736e