tools: correct tools/tests/depriv/Makefile

tools: correct tools/tests/depriv/Makefile

tools/tests/depriv/Makefile directly builds the target program from
its C-source. This is problematic when an incremental build is needed
after a header the program is depending on has been modified: in this
case all headers are added into the gcc call and the build will fail.

Correct that by adding a rule for building the program from its .o
file.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>