Do not include SDL_mixer.h from headers

In case the header is included but not used by a program compiled without
sound support.

src/platform/C4SoundLoaders.cpp

@@ -217,6 +217,9 @@ VorbisLoader VorbisLoader::singleton;
 #endif
 
 #ifdef HAVE_LIBSDL_MIXER
+#define USE_RWOPS
+#include <SDL_mixer.h>
+
 bool SDLMixerSoundLoader::ReadInfo(SoundInfo* result, BYTE* data, size_t data_length, uint32_t)
 {
 	// Be paranoid about SDL_Mixer initialisation

src/platform/C4SoundSystem.cpp

@@ -35,6 +35,11 @@
 #include <C4Viewport.h>
 #include <C4SoundLoaders.h>
 
+#ifdef HAVE_LIBSDL_MIXER
+#define USE_RWOPS
+#include <SDL_mixer.h>
+#endif
+
 using namespace C4SoundLoaders;
 
 C4SoundEffect::C4SoundEffect():

src/platform/C4SoundSystem.h

@@ -40,9 +40,7 @@ class C4SoundInstance;
 #include <fmod.h>
 typedef FSOUND_SAMPLE* C4SoundHandle;
 #elif defined(HAVE_LIBSDL_MIXER)
-#define USE_RWOPS
-#include <SDL_mixer.h>
-typedef Mix_Chunk* C4SoundHandle;
+typedef struct Mix_Chunk* C4SoundHandle;
 #elif defined(USE_OPEN_AL)
 #ifdef __APPLE__
 #import <OpenAL/al.h>