sdl_mixer: drop custom target install

Fixes http://autobuild.buildroot.net/results/6ba/6ba399db1715a7759048e0d532768a335c3c97cc/



The default rule (make install) works fine, even for static build - so use
that instead of explicitly copying *so files.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>