fixup pandora compilation with SDL < 1.2.12

This was checked to compile (but not link of course) on my system,
using cmake with pandora enabled, and pointed to the SDL_mixer
1.2.0 header.
Merge branch 'master' of https://github.com/wesnoth/wesnoth
Merge branch 'master' of https://github.com/wesnoth/wesnoth