mpd: fix build failures with gcc 4.9.x

As reported by Sagaert Johan in the mailing list mpd 0.18.x fails to
build with gcc 4.9.x
Patch status: sent upstream.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>