media-libs/libsdl2 didn't have ppc keyword a while back.
Now it's stable on ppc, so adjust the p.use.mask entry.
# Ian Delaney <idella4@gentoo.org> (04 Jan 2016)
# on behalf of Ilya Tumaykin <itumaykin+gentoo@gmail.com>
-# media-libs/libsdl2 lacks ppc32 keyword. See bug #508226.
-media-video/mpv sdl
+# media-libs/libsdl2 lacked ppc32 keyword at the time. See bug #508226.
+# Ready for the next stable mpv version after the one below.
+=media-video/mpv-0.18.0-r1 sdl
# Anthony G. Basile <blueness@gentoo.org> (26 Oct 2015)
# Mask x265 on vlc for bug #564138