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 <patrick@stwcx.xyz>
Change-Id: I4548813cba19822c1367c3de1b81cc2f034518c6
1 file changed