package/liquid-dsp: fix legal-info

package/liquid-dsp: fix legal-info

The license file is named LICENSE, not COPYING.

Fixes:
    http://autobuild.buildroot.org/results/c82/c82baa5549bdca405e0f077c7ab8c77dcec9a582/
    http://autobuild.buildroot.org/results/9c7/9c76ce4f9d69db1aa67cc9c07e4c5c2e5e8d93e0/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: guillaume william brs <guillaume.bressaix@gmail.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>