build-unit-test-docker: add nlohmann-json dep to sdbusplus

Pending commit[1] adds nlohmann-json usage in sdbusplus;
update docker dependencies to reflect that.

[1]: https://gerrit.openbmc.org/c/openbmc/sdbusplus/+/74858

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9027c06877c35ae9c5b130d1ce0bec881060b5d7
1 file changed