build-unit-test-docker: add libunifex
build-unit-test-docker: add libunifex

unifex is a library that sdbusplus will begin to depend on in order to
support co-routines.  Add it here to match the recipe that has been
submitted upstream.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I4548813cba19822c1367c3de1b81cc2f034518c6

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts