libsoxr: add a hash file

It will clarify errors like this:
  http://autobuild.buildroot.net/results/923/923d4ba849839beca89f427687bbd903d6037b15/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>