package/sdl_sound: fix constness in debug mode

package/sdl_sound: fix constness in debug mode

When in debug mode, ./configure forces -Werror, which uncovers a
constness issue in the speex decoder.

Add a patch to fix this.

Fixes:
    http://autobuild.buildroot.net/results/6f5/6f5e1b782923d6e69e929466f0ddd3eeba28c6f1/
    http://autobuild.buildroot.net/results/c1b/c1b31dc37fe62cb6901edad02bf50d73072b7cf0/
    http://autobuild.buildroot.net/results/162/162e6d77e5812060001ea22faff99055aae3122a/
    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>