Makefile: Add missing library to the dp_opt_tests
    
    dp_opt_tests cannot be linked properly if link_all_deplibs is disabled (debian)
    
    /usr/bin/ld: src/tests/cmocka/dp_opt_tests-test_dp_opts.o:
        undefined reference to symbol 'poptFreeContext@@LIBPOPT_0'
    /usr/bin/ld: note: 'poptFreeContext@@LIBPOPT_0' is defined in DSO
    /lib64/libpopt.so.0 so try adding it to the linker command line
    /lib64/libpopt.so.0: could not read symbols: Invalid operation
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 5c36e1f8901a4baff2b51d81d87c2b577f84fef6)