buildroot

mpg123: install to staging

mpg123 also provides a shared library (libmpg123.so), so install into
staging so other programs can find it.

[Peter: reworded commit message]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>