Add nlohmann-json to target toolchain to support building the speech
services. This is required until either the agl-speech-framework
feature is added as a dependency of agl-demo, or the speech services
are migrated into the core profile.
Bug-AGL: SPEC-2856, SPEC-2857
Change-Id: I67fe96a73f5cccc8c2dbf2642041f2b64802c512 Signed-off-by: Scott Murray <scott.murray@konsulko.com>