mpg123: fix module loading with libtool / !HAVE_DEVFILES

mpg123 enables loadable modules support if libtool is detected, but
by default tries to load <module>.la, which we strip from the
rootfs unless HAVE_DEVFILES is enabled.

Fix it by directly loading .so files instead.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>