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>

package/multimedia/mpg123/mpg123.mk

@@ -8,6 +8,7 @@ MPG123_VERSION = 1.13.1
 MPG123_SOURCE = mpg123-$(MPG123_VERSION).tar.bz2
 MPG123_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/mpg123
 MPG123_CONF_OPT = --with-optimization=0 --disable-lfs-alias
+MPG123_INSTALL_STAGING = YES
 MPG123_CPU = $(if $(BR2_SOFT_FLOAT),generic_nofpu,generic_fpu)
 
 ifeq ($(BR2_arm),y)